Per-database permissions

[TheSlimvReal]

After #767 every entity will be in its own database. Each database can have its own permission based on user roles. This should be used to restrict the access to some databases for certain user groups. An array of user groups can be set as permittedGroups to the config of a entity. This should trigger a change to the permission settings of the according couchDB database. After that, only users of the specified groups will be able to read from an write to this database.
Users without permissions will not be able to synchronize with the remote database.

[TheSlimvReal]

Closed in favor of #956