Removed GDCR from titlepage.
Added a hint indicating where the original of the file is kept.
New groups now get a default color (pink, in the hope that everybody will change it...). It is non-trivial to add a proper frontend validator, so we only add backend validation for the color field.
Adapted a test, added a new one.
Merge pull request #844 from NicoleRauch/master

Removed GDCR and improved color checking of groups