Fixes: #486

Accept all expressions that croniter accepts.
If cron-descriptor throws an exception, don't show the
description to the user.

Fixes: #472

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: #467

Fixes: #202

Fixes: #428

Fixes: #465

Fixes: #450

Fixes #455

Fixes #282

When sending email using Django's default email
backend (SMTP), and if there is a network issue, the backend
can throw SMTPServerDisconnected.

This commit adds a retry logic which retries sending the
email two times when SMTPServerDisconnected is thrown.

Normally, when a webhook call fails (timeout, connection
error, non-2xx response), the HTTP request is retried up to two
times (so up to 3 times total). This is useful when sending
actual notifications, in case the webhook target has a temporary
glitch.

When interactively testing a webhook integration
("Send Test Notification" in the
"Integrations" page), we would prefer to see any errors ASAP
on the screen instead of retrying and so possibly swallowing them.

One specific use case is webhook targets that take long time to
generate a response. "Send Test Notification" is synchronous,
meaning that the user could be stuck for
5 x 3 = 15 seconds waiting for the  test HTTP request to time out
three times.

Fixes: #429

Fixes: #442

Fixes: #440

Fixes: #436

Fixes: #435