[FEATURE] Option to purge unused mods

[sylae]

Is your feature request related to a problem? Please describe.
Oftentimes we remove or change mods as things get updated or we want to try something else. Especially with total-conversion campaigns like OPTRE or TIOW, you're looking at swapping 100+ mods at a time. It'd be nice to have a button that'll remove symlinks and purge mod folders for mods that arent included in any server config.

Describe the solution you'd like
A button on the "mods" tab, complementary to the "delete all" button, which will delete and unlink any mods that aren't in at least one server configuration.

Describe alternatives you've considered
The only reasonable way to remove unused mods is to remove all mods, and then re-add and re-download all the ones you actually want to keep. Alternatively, picking through the list while cross-referencing every server config to verify which ones are used and which aren't.

[jupster]

Enhanced version of #89