Cherrypick from opengever.core release 4.0.

Use correct content-type when returning 'OK' responses.

Conflicts:
	opengever/base/utils.py
	opengever/task/browser/accept/utils.py

(cherry picked from commit b00eabf64587017954a229e70708bbff6c6d5915)
Referenceprefix manager: Removed wrong space in unlock link.

(cherry picked from commit 890b0f85748fff7b6e19d70bbd3ef72ad9905ffa)
Referenceprefix manager: Fixed KeyError when repository is already removed.

(cherry picked from commit 3e77f57168e6417d8278b6fed8532a8897a55218)

Conflicts:
	docs/HISTORY.txt
	opengever/repository/tests/test_reference_prefix_manager.py
Updated CL.