[groups] unsubscribe group membership

[Former user]

Reported by sven: If a user will not even longer be a member of a group. How can he unsubscribe? I think we need a methode for unsubscribing from a group. Maybe next to the group name in the group list at the settings page http://www.bibsonomy.org/settings

[Robert Jäschke]

another group task :)

[Daniel Zoller]

Currently this is only allowed for group admins to remove a member from a group (see SettingsHandler). Please note, although it is currently not enabled a post can be assigned to multiple groups and not to only one. The SettingsHandler currently assumes that there is only one group per post and only sets the group to private.

[Thomas Niebler]

Partially resolved with cset 9210b2b (migrated SettingsHandler to DBLogic)