Fix unicode-decode error when syncing tasks.

(cherry picked from commit 3753bd43f64f6e0f2dd2522481cbe9cbfb15ab3e)

Conflicts:
	opengever/task/deadline_modifier.py
Referenceprefix manager: Made prefix-mappings of already delete objects removable.

(cherry picked from commit 40016cc42f7c279a27d5a070605edf1d4a04352d)
Merge pull request #576 from 4teamwork/pg_3.4.1

Additional backports for the 3.4.2 Release