Channel.last_notify is the datetime of the most recent
notification sent via the channel. Channel.last_error is
the error message (blank if the delivery was successful).

In the integrations list, "Last Notification" column,
use these fields instead of looking up the most recent
Notification object. This saves some db queries,
and also fixes a subtle issue: if prunenotifications
cleans up all notifications for a given channel, the
"Last Notification" column would display "Never", which
would not be correct – not any more.

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

Fixes: #512

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: #428