Localization for frontend (validators).
Localization of error messages in activities.
Moved two methods from activitiesCoreAPI down to activitystore.
General cleanup:
- Persistence now only returns the error code on save, nothing else
- Stores now only return the error code on save, not the object to be saved
- This simplifies the API save methods.

Also: Removed persistence.saveValueObject + test
Merge pull request #546 from NicoleRauch/master

Letzte Reste Frontend-Lokalisierung + allgemeine Aufräumarbeiten