Redirect unauthenticated users to the sign in page
instead. Rationale:

- The content on the welcome page is what often belongs
  to a separate "marketing site". The marketing content
  is of no use on self-hosted instances, which typically
  have new signups disabled and are for internal use only
- (the real reason, let's be honest) a number of
  self-hosted instances are accessible over the public
  internet. Search engines index the nearly identical
  landing pages and see them as duplicated content.

Fixes: #281

cc: #556

Channel.last_notify is the datetime of the most recent
notification sent via the channel. Channel.last_error is
the error message (blank if the delivery was successful).

In the integrations list, "Last Notification" column,
use these fields instead of looking up the most recent
Notification object. This saves some db queries,
and also fixes a subtle issue: if prunenotifications
cleans up all notifications for a given channel, the
"Last Notification" column would display "Never", which
would not be correct – not any more.

The jsonify decorator parses request payload as JSON
and puts it in request.json. The payload would normally
be a complex object, but if a client sends, let's say,
a single integer, then request.json is a python int.

The authorize decorator looks for API key first in request
headers, then in request body. It expects the request
body to be a complex object.

This commit changes adds the following validation rule in
the jsonify decorator: if request body is not empty, it
*must* parse as JSON, and the root element of the parsed
document *must* be a dict.

Fixes: #574

With a catch-all "except:" rule, we would swallow any unexpected
exceptions (ValueError, etc.) in cronsim. But we want to know
about them. cron_preview is a place where we can afford to crash, and generate a crash report.

Previous retry logic was:
- max 3 tries
- every try times out after 5 seconds

The new retry logic is:
- max 3 tries
- every try times out after 10 seconds
- if the first two tries have used > 10 seconds, don't
  do the third try

cc: #569

Fixes: #566

In the Details page, if the user click "Ping Now",
and the project is using {ping-key}/{slug} URLs,
but the ping key is not set, then show a
"Ping Key Required" message instead of trying to ping
and invalid URL.

1. Drop API support for GET, DELETE requests with a request body.
Healthchecks had an undocumented quirk where you could authenticate a
GET or DELETE request by putting a '{"api_key":"..."}' in request body.
This commit removes this feature.

Note: POST requests can still authenticate either by sending
a X-Api-Key header, or by putting a "api_key" key in request body.
GET and DELETE requests can now only authenticate with the
request header.

2. Add missing @csrf_exempt annotations in API views
When client sends a HTTP POST request to a GET-only endpoint,
the server is supposed to respond with "405 Method Not Allowed".
Due to CSRF checking, a couple endpoints were responding with
"403 Forbidden" instead. Adding @csrf_exempt annotations fixes
the problem.

In a project with ~300 checks,

* HTML size (uncompressed) before: 772KiB
* HTML size (uncompressed) after: 703KiB

In SMS, Signal and WhatsApp forms, reject the form if
user unchecks both "alert when check goes DOWN" and
"alert when check goes UP".

cc: #552

Fixes: #77