Update database relationship diagram

[michplunkett]

Fixes issue

https://github.com/lucyparsons/OpenOversight/issues/683

Description of Changes

This PR updates the database relationship diagram to include the changes made over the last couple of months.

The image was created using this: https://www.jetbrains.com/help/datagrip/creating-diagrams.html#db_diagrams

Tests and linting

• [x] This branch is up-to-date with the develop branch.
• [x] pytest passes on my local development environment.
• [x] pre-commit passes on my local development environment.

[michplunkett]

Hmmm, that would be v cool. I'll take a look and see if I can find something that could do that.