Make events/event suggestions lockable

[HerrLevin]

Is your feature request related to a problem? Please describe.
When two admins/moderators are modifying an event or event suggestion, it will be likely that they'll edit the same item, which might lead to issues/duplications

Describe the solution you'd like
Add a (un)locking mechanism to reduce possible conflicts.

Additional context:
I've imagined, adding lockedAt and lockedBy fields to both the event table and event suggestions table. Both should only be savable by the user that's currently saved in the lockedBy-column. After a 10/15(?)-Minute timeout AND after a save, the lock should be removed. I'd suggest a check for unlucking once another user tries to edit the item, rather than a cron, trying to unlock since this would increase load for something that'll rarely is an issue.