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.

cc: #547

In a project with ~300 checks,

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

cc: #547

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.

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

Fixes: #512

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

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: #467

Fixes: #428

Fixes: #465

Fixes #455

Fixes #282