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 :-)

Planning to use it for sensitive operations (add/remove security keys),
change email, change password, close account.

The decorator sends a six-digit confirmation code to user's email
and renders a form for entering it back. If the user enters the
correct code, the decorators sets a sudo=active marker in
user's session, valid for 30 minutes.

The team size limit is applied to the number of distinct users across all projects. Fixes #332.