Retry e-mail notifications on certain types of failures

## What?

It should be possible to enable retrying sending e-mail notifications when certain conditions (e.g. `java.net.SocketTimeoutException`) occur. Ideally the retry limit would be configurable, with an increasing backoff between attempts.

## Why?

Temporary network conditions or mail server unavailability may lead to failed notifications. It would be preferable to potentially receive duplicate notifications (due to retries for perceived delivery issues) than not receive them at all.

Also-- the system event and notification that current occurs on a e-mail delivery failure can be tricky to tie back to the associated alert/event.

## Your Environment

* Graylog Version: 5.1.4