cc: #552

This commit adds a {% absolute_site_logo_url %} template tag.
The tag emits an absolute url pointing to either
SITE_LOGO_URL or to the fallback picture.

The tag is used in base email template, in slack message
template, and in "Add MS Teams" page.

This commit also fixes a couple instances where absolute URLs
were constructed like so:

    {% site_root %}/docs/

This would result in incorrect links if Healthchecks is not
running at webserver's root. The correct way is:

    {% site_root %}{% url 'hc-docs' %}

Finally, this commit removes stuff/logo.svg and
stuff/logo-full.svg. Selfhosted sites should not use the
official Healthchecks.io logos, so no point keeping them around
there.

Fixes: #539

It is obsoleted by Profile.reports

Fixes: #512

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

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.

Fixes: #472

Problem: if you use uBlock Origin, and enable the
"Fanboy's Social" filter list, Healthchecks does not show
Telegram or WhatsApp icons. This is because the filter list
contains "##.icon-telegram" and "##.icon-whatsapp" entries.

This commit changes the CSS class prefix to "ic-". So we're
now using icon classes like "ic-telegram" and "ic-whatsapp".

As a bonus, we save 2 bytes in HTML per displayed icon :-)

Fixes: #202

To send notifications, sendalerts calls Flip.send_alerts().
I updated Flip.send_alerts() to be a generator, and to yield
a (channel, error, send_time_in_seconds) triple per sent
notification.