Now all activities are colored by their group colors in the calendar - no more special colors for activities.
Each activity now displays by whom it was created.
We like dots :-)
Some opportunities for cleanup, as pointed out by leider
Merge pull request #512 from NicoleRauch/master

Removed special activity coloring and added display of activity creator.
When editing an activity, one can determine whether its resources are open for registration or not. The information is persisted as well.
Minor improvement of comment
Beautification of the edit page.
Subscription now checks whether the registration is open or not. It gives an error message if not.
Now registration status for the resources is indicated on the activity page.
Application tests for registration that is not open.
Oops, we need to invoke the callback in case of an error...
Merge pull request #513 from NicoleRauch/master

Jetzt kann man die Registrierung für eine Ressource sperren und freigeben.