For example, if we need to use a custom From: address,
we can now do:

    m = make_message("template-name", recipient, ctx)
    m.from_email = "...."  # customize here
    send(m)

- add optional `from_email` argument
- add test cases that exercise the retry loop

Fixes: #354

Fixes: #323

Fixes: #539

It is obsoleted by Profile.reports

This is to help troubleshoot 2FA issues without
running manual SQL queries.

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

This is in preparation of adding an option for weekly
reports (#407)

Fixes: #512

When an account has a password, replace "Set Password"
button's label with "Change Password"

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: #497

Profile.next_nag_date tracks when the next hourly/daily reminder
should be sent. Normally, sendalerts sets this field when
a check goes down, and sendreports clears it out whenever
it is about to send a reminder but realizes all checks are up.

The problem: sendalerts can set next_nag_date to a non-null
value, but it does not clear it out when all checks are up.
This can result in a hourly/daily reminder being sent out
at the wrong time. Specific example, assuming hourly reminders:

13:00: Check A goes down. next_nag_date gets set to 14:00.
13:05: Check A goes up. next_nag_date remains set to 14:00.
13:55: Check B goes down. next_nag_date remains set to 14:00.
14:00: Healthchecks sends a hourly reminder, just 5 minutes
       after Check B going down. It should have sent the reminder
       at 13:55 + 1 hour = 14:55

The fix: sendalerts can now both set and clear the next_nag_date
field. The main changes are in Project.update_next_nag_dates()
and in Profile.update_next_nag_date(). With the fix:

13:00: Check A goes down. next_nag_date gets set to 14:00.
13:05: Check A goes up. next_nag_date gets set to null.
13:55: Check B goes down. next_nag_date gets set to 14:55.
14:55: Healthchecks sends a hourly reminder.

Fixes: #486