Add migration for custom ACL actions

[owi92]

Custom actions for events are now stored in DB with a new column. This is only the backend side of things, so they won't show up in the ACL UI yet.

Part of #1004

[LukasKalbertodt]

Looks good overall.

I think using jsonb as DB storage is fine. It is unlikely that we ever need to filter by these values in a DB query: from the DB perspective it's one atomic value. And even if requirements change, PostgreSQL can apparently index jsonb fields somehow, or at worst case we have to change the DB schema, but that's all possible. So going with the most straight forward way seems like a good idea.

Apart from a few minor remarks below, generally this PR could use some more comments. A short doc comment on CustomActions struct explaining what the hash map represents (mapping action to list of roles). Same at top of the DB migration. And maybe a comment for check_custom_actions_format. None of those need to be long, but yeah, neither jsonb nor HashMap<String, Vec<String>> make it immediately clear what data to expect how.

[owi92]

Thanks for the review. Here's a little screenshot of an event with custom actions in the db, since I don't expect you to test this (would require a bit of tinkering in your local Opencast):

(Should have included that in the first place)