Repositoryfolder referenceprefixex manage-form needed.

The referenceprefix functionality in GEVER allot that all prefixnumbers, wich are used   at on time for a repositoryfolder, could not be reused by another repositoryfolder. Neither when the prefixnumber isn't used anymore by another repositoryfolder.

This constraint generates some troubles and support costs in the past. Therefore a referenceprefix manage form should be implemented, which allows users with the administrator role to unlock no longer needed prefixnumbers.

Todo:
* A form which list all prefixes registered for the actual context, it should be possible to delete no longer used prefixes.
* Action `Manage prefixes` on every repositoryfolder
* The action should only be visible for uses with edit permission on the current repositoryfolder.
* All changes in this form should be logged in the repository journal