Repositoryfolder referenceprefixex manage-form needed.

:construction_worker:  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

Documentation:

* Added action in repository folder
![screen shot 2013-10-08 at 11 57 18](https://f.cloud.github.com/assets/1375745/1287874/d488dc6a-3000-11e3-8eb3-5b8c4a0bc456.png)

* Added view for prefix manager
(prefix 1.7 was changed to 1.99 which leaves 1.7 unused)
![screen shot 2013-10-08 at 11 59 28](https://f.cloud.github.com/assets/1375745/1287876/e169fac2-3000-11e3-8afe-ad221c09179a.png)