Fixes: #202

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.

The PhoneNumberForm is used in "Add SMS" and "Add Phone Call" pages.
The PhoneUpDownForm is a subclass of PhoneNumberForm and
adds "up" and "down" boolean fields. It is used in "Add Signal"
and "Add WhatsApp" pages.

Fixes: #428

Fixes: #465

Fixes #455

Read-only users cannot pause checks.