Accept group invite

[Mario Holtmüller]

When a user accepts a group invite and the group member settings are set to private, the user who accepts the invite lands on an "Access denied" page.

[Daniel Zoller]

group role privileges do not depend on database id anymore, now the compiler checks it

if group privlevel is hidden the user now always sees his own group membership (maybe fixes #2385)

todos and cleanup

→ <<cset 80c66bce6c20>>

[Daniel Zoller]

please check