Fixes: #539

Fixes: #536

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"

Fixes: #499

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

See also: #483

Not all email clients are formatting the `ping.body` contents uniformly. Even using different applications from the same email provider results in a different display of the `ping.body` contents. There are two basic issues:
* Not all email clients are honoring the fixed-width font that should be used inside `<pre>` tags. Using fixed-width font is listed in the definition on https://www.w3schools.com/tags/tag_pre.asp
* Not all email clients are displaying the text with a 1em line height. This was a recent change to the healthchecks WebUI in 9fd9c8e4ef but is not part of the definition of the `<pre>` tag. I'd like to add this to the emails to make Healthchecks more uniform between the website and the email notification.

Gmail Webmail:
- [x] Is using fixed-width font
- [ ] Line height is set by the webmail client to 18px
Gmail Android App:
- [ ] Text is not fixed-width
- [ ] Line height has extra padding

ProtonMail Webmail:
- [x] Is using fixed-width font
- [x] Line height is correct
ProtonMail Android:
- [ ] Text is not fixed width
- [ ] Line height has extra padding

The testing I performed is not extensive, but it does show how multiple clients are displaying the contents differently. To make the display of the `ping.body` more uniform I'd like to add a bit of formatting information to the `<pre>` tag.

Fixes: #486

so you can see whether it's up or down at first glance

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.