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: #550

cc: #547

cc: #547

cc: #547

If user has both WebAuthn and TOTP configured,
when logging in, they will be asked to choose between
"Use security keys" and "Use authenticator app".
The "Use authenticator app" is a link to a different
page (/accounts/login/two_factor/totp/). This commit makes
sure the ?next= query parameter is preserved when navigating
to that page.

For reference, the ?next= query parameter is the URL we should
redirect to after a successful login. Use case:
User is logged out. They click on a bookmarked "Check Details"
link. They get redirected to the login form. After
entering username & password and completing 2FA,
they get redirected to the "Check Details" page they
originally wanted to visit.

This has dual purpose:

* if user has both WebAuthn and TOTP set up, they can choose
  between the two as equal options.
* we initiate WebAuthn flow only after an explicit user action
  (button press). This may help with authentication failures
  on recent MacOS, iOS and iPadOS versions [1]

[1] https://support.yubico.com/hc/en-us/articles/360022004600-No-reaction-when-using-WebAuthn-on-macOS-iOS-and-iPadOS

Fixes: #354

Fixes: #323

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.