I think this is finally ready for a first pair of extra eyes. I still have to go over it a bit more carefully, but having someone else look at it will help find any issues.

Main changes:

• New events targeting VC rooms and associations. I didn't go as far as replacing the whole plugin inheritance hell with it, but that should be the direction for future plugins;
• Added a proper VCRoom.delete method which deals with the logic of deleting a whole room and its associations
• Changed session_updated and session_block_updated to allow for a changes dict to be passed
• Made split_log_location_changes non-destructive, since object mutation was messing with some tests
• A few UI changes to use Semantic UI and custom elements instead of legacy CSS when rendering almost all VC-related stuff
• Session delete operation changed in order to make signal logic more usable
• Event test fixture: give creator management access by default (as is the case in prod)

Unrelated things which are bundled too:

• Serialize locators as dicts and add clearer repr
• Add format_repr to VCRoomEventAssociation

PRs for the CERN and general-purpose plugins will follow.