The initial implementation was just calling signal-cli directly
using `subprocess.run`.

Going with DBus makes it easier to shield signal-cli from the
rest of the system. It also makes sure the signal-cli daemon is
running in the background and receiving messages. This is important
when a recipient does the "Reset secure connection" from the app. We
must receive their new keys, otherwise our future messages will
appear as "bad encrypted message" for them.

Fixes: #428

* Add HTTP header authentiation backend/middleware

* Add docs for remote header auth

* Improve docs on external auth

* Add warning for unknown  REMOTE_USER_HEADER_TYPE

* Move active check for header auth to middleware
Add extra header type sanity check to the backend

* Add test cases for remote header login

* Improve header-based authentication

- remove the 'ID' mode
- add CustomHeaderBackend to AUTHENTICATION_BACKENDS conditionally
- rewrite CustomHeaderBackend and CustomHeaderMiddleware to
use less inherited code
- add more test cases

Co-authored-by: Pēteris Caune <[email protected]>

The default port after following the directions is actually 8000 instead of 8080.

Since anonymous checks aren't created any more, don't document the manage command to clean them up.

This change allows SMTP configuration via email, but users can
still choose to configure the settings directly in local_settings.py

pip install will fail when you cannot compile some of the dependencies.
one is gcc the other is the Python.h


```
Building wheels for collected packages: rcssmin, rjsmin
  Running setup.py bdist_wheel for rcssmin ... error
  Complete output from command /home/ubuntu/webapps/hc-venv/bin/python3 -u -c "import setuptools, tokenize;__file__='/tmp/pip-install-ipfho29k/rcssmin/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" bdist_wheel -d /tmp/pip-wheel-u0q6mggl --python-tag cp36:
  running bdist_wheel
  running build
  running build_py
  creating build
  creating build/lib.linux-x86_64-3.6
  copying ./rcssmin.py -> build/lib.linux-x86_64-3.6
  running build_ext
  building '_rcssmin' extension
  creating build/temp.linux-x86_64-3.6
  x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -DEXT_MODULE=_rcssmin -UEXT_PACKAGE -I_setup/include -I/usr/include/python3.6m -I/home/ubuntu/webapps/hc-venv/include/python3.6m -c rcssmin.c -o build/temp.linux-x86_64-3.6/rcssmin.o
  unable to execute 'x86_64-linux-gnu-gcc': No such file or directory
  error: command 'x86_64-linux-gnu-gcc' failed with exit status 1
  
  ----------------------------------------
  Failed building wheel for rcssmin
  Running setup.py clean for rcssmin
  Running setup.py bdist_wheel for rjsmin ... error
  Complete output from command /home/ubuntu/webapps/hc-venv/bin/python3 -u -c "import setuptools, tokenize;__file__='/tmp/pip-install-ipfho29k/rjsmin/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" bdist_wheel -d /tmp/pip-wheel-axnaq3w9 --python-tag cp36:
  running bdist_wheel
  running build
  running build_py
  creating build
  creating build/lib.linux-x86_64-3.6
  copying ./rjsmin.py -> build/lib.linux-x86_64-3.6
  running build_ext
  building '_rjsmin' extension
  creating build/temp.linux-x86_64-3.6
  x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -DEXT_MODULE=_rjsmin -UEXT_PACKAGE -I_setup/include -I/usr/include/python3.6m -I/home/ubuntu/webapps/hc-venv/include/python3.6m -c rjsmin.c -o build/temp.linux-x86_64-3.6/rjsmin.o
  unable to execute 'x86_64-linux-gnu-gcc': No such file or directory
  error: command 'x86_64-linux-gnu-gcc' failed with exit status 1
```

```
  Running setup.py bdist_wheel for rjsmin ... error
  Complete output from command /home/ubuntu/webapps/hc-venv/bin/python3 -u -c "import setuptools, tokenize;__file__='/tmp/pip-install-cfntw7bo/rjsmin/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" bdist_wheel -d /tmp/pip-wheel-ytqxu9_b --python-tag cp36:
  running bdist_wheel
  running build
  running build_py
  creating build
  creating build/lib.linux-x86_64-3.6
  copying ./rjsmin.py -> build/lib.linux-x86_64-3.6
  running build_ext
  building '_rjsmin' extension
  creating build/temp.linux-x86_64-3.6
  x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -DEXT_MODULE=_rjsmin -UEXT_PACKAGE -I_setup/include -I/usr/include/python3.6m -I/home/ubuntu/webapps/hc-venv/include/python3.6m -c rjsmin.c -o build/temp.linux-x86_64-3.6/rjsmin.o
  In file included from rjsmin.c:18:0:
  _setup/include/cext.h:34:10: fatal error: Python.h: No such file or directory
   #include "Python.h"
            ^~~~~~~~~~
  compilation terminated.
```