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.

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.

In a project with ~300 checks,

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

PagerDuty Connect is deprecated and will be discontinued.
It is replaced by PagerDuty Simple Install Flow (see
README for setup instructions).

Fixes: #512

Cannot use functools.cached_property, as it was added in Py 3.8,
but we support 3.6+

Primary sort key: projects with overall_status=down go first
Secondary sort key: project's name

Fixes: #486

If the notification does not exist, or is more than a hour
old, return HTTP 200 (instead of 400 or 404) so the other
party doesn't retry over and over again.

Accept all expressions that croniter accepts.
If cron-descriptor throws an exception, don't show the
description to the user.

Fixes: #467

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: #348

Fixes #282