Add notifications when a translation or source unit is deleted

[superDross]

Add notifications when a translation or source unit is deleted

The notification occurs prior to the actual deletion of the Unit. This is because the Change object and Unit object are connected by a foreign key. As such, we cannot notify after the deletion event.

Proposed changes

As above, specifically for pushing deletion messages to Fedora messaging.

Checklist

• [x] Lint and unit tests pass locally with my changes.
• [x ] I have added tests that prove my fix is effective or that my feature works.
• [x ] I have added documentation to describe my feature.
• [x ] I have squashed my commits into logic units.
• [x ] I have described the changes in the commit messages.