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 :-)

Previously, I had changed the default value to "", to force
users to set the SECRET_KEY value (the app refuses to start
if SECRET_KEY is empty).

The problem with that is, out of the box, with the default
configuration, the tests also don't run and complain about the
empty SECRET_KEY.

So, a compromise: revert back to the default value "---".
At runtime, if SECRET_KEY has the default value, show a  warning
at the top of every page.

Fixes: #467

Fixes: #202

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: #465

Need it to escape quirks mode in email clients.

Fixes: #347

Fix simple typo in docs, libary -> library

There is a small typo in templates/docs/python.md.

Should read `library` rather than `libary`.

Fixes: #348

Fixes: #450

Move terraform-provider-healthchecksio to the "API Wrappers"
category, which is more appropriate than "Tools for Self-Hosting".

Move terraform-provider-healthchecksio to the "API Wrappers"
category, which is more appropriate than "Tools for Self-Hosting".

Fixes #455

Fixes #282