cc: #547

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

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

When an account has a password, replace "Set Password"
button's label with "Change Password"

The line height in the ping request body is being set by the bootstrap.css body tag to 1.42857143. The additional line height makes request bodies with unicode characters have spacing between lines that shouldn't be there.

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