SeAT Manager

Replaces #546, updating it to the new modular service format.

I would consider this manager Beta at the moment. Backup your SeAT database before activating this, and regularly after in case something goes completely haywire. Watch your logs for errors on both applications.

It does work but the error handling is rather lax. It also needs to have unit testing added, but as I know a lot of people have been waiting for this service for some time, I'm going to let it slide for now. Edit: I added some unit tests because I couldn't stomach the -2.3% drop in coverage 😦

Thanks to @westonmyers for helping test this.