Allow users editing their membership dates.

But do not allow to change their status.
Add ability spec
Add test for group memberships set into the future
Prevent changes to ancestor_id and other integral values
Merge pull request #37 from fiedl/jr/531-berechtigungen-zur-corporatevita

Jr/531 berechtigungen zur corporatevita
adding flamegraph.

to view the flamegraph, just add the url parameter ?pp=flamegraph.