[Bug]: Use MLPromise in all UI-facing methods

[tmolitor-stud-tu]

We should update all MLMucProcessor methods that are UI-facing to use the new MLPromise framework rather than the old and inflexible addUIHandler:forMuc: UI-handler framework.

The already updated method publishAvatar:forMuc: can be used as blueprint for this.

All methods calling callSuccessUIHandlerForMuc: should be updated to use the MLPromise framework. Potentially bubbling this change up to the UI-facing method called from outside of the MLMucProcessor.

Nearly all UI-facing methods call promisifyMucAction() wrapper around the UI-handler framework.