Fixes: #281

In a project with ~300 checks,

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

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).