Prepare event invitation handling for migration to REST

What does this MR do?

• Unify the various event XHR calls into one, eventresponse
• Extract all of those calls to helper functions that are easily replacable
• Consolidate addInviteStatus into setInviteStatus
• Much more readable option menu construction (at the expense of "clever" handling that was actually bugged)
• Use event permissions where applicable
• Implement translations everywhere I stumbled across hardcoded German text
• Implement translator into EventView constructor (is there a better way?)

How confident are you it won't break things if deployed?

• Quite confident, actually. Tested all possible UI paths locally.

Links to related issues

• &9

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. Create events, invite users/regions
2. Respond to invitations from your dashboard
3. Respond to invitations from the event page
4. Change your mind later on the event page

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• add a short text that can be used in the release notes (forDev)
• Once your MR has been merged, you are responsible to create a testing issue in Beta Testing Repo:
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...). how this change can be tested.
  • Be aware, that also non technical people should understand.