Add/update matrix room to event

[friedger]

Users should be able

• to add a new matrix room to a new or existing event
• to remove the matrix room of an existing event
• to be able to join the matrix room of an event from the event details

This requires

• to add action that creates a new matrix room using matrix-js-sdk. The room should be federated, anybody with link can join. If the corresponding event is private the room should be private, if the event is public, the room should be public and use the same name as the event. The room should use end-to-end-encryption(!)
• to add action to delete matrix room
• to add action to update private/public property of matrix room

The UI has already been added in #64

[friedger]

UI was done in PR #64 (issue #60)

[friedger]

New rooms are created for events now.

[friedger]

delivered in branch develop