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

Minor API documentation edits.