In the Details page, if the user click "Ping Now",
and the project is using {ping-key}/{slug} URLs,
but the ping key is not set, then show a
"Ping Key Required" message instead of trying to ping
and invalid URL.

cc: #547

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 :-)

Read-only users cannot pause checks.