Refactored the web-interface for group-creation.

Restructured the web-interface for the creation of a group entity.
Multiple entity-creation is replaced by single creation. Now it is also allowed
to create a group with the role 'ROLE_ADMIN', which implies that the
logged in user will be the group-leader. The automatic creation of a
group-leader in case od beeing 'SUPERADMIN' has been moved to a private
method.
API-Docs and whitespace. Non-functional change.
Refactored the web-interface for updating groups.

Removed updating multiple group-instances. The web-service now expects a
single group to be updated.
Also allowed admin-roles (group-leaders) to update groups.
Secured the update-group mechanism for admins.

Ensured that in case of having an admin, no super-admin, as logged in user
the update mechansim for a group will only work if the user is meber of
this group.
Easier detection of a possible admin role of a user.

Added a method for the detection of an admin-role within the User's set of
roles to the User-class.
Used the new method in create and update methods for groups.
Refactored the web-interface for deleting groups.

Ensured that admins (group leaders) can delete their own groups.
New web-interface for reading all own groups.

Implemented a web-interface in order to read all own group objects. In
case of beeing SUPERADMIN all groups are returned.
Improved create and update of groups.

Extended the create and update-interfaces for groups in order to allow
passing user-IDs which will be added to the group.
Therfore the group-model-class gets a field grantedUsers which is a list
of integers (user-IDs) which are transformed to real user objects in the
service level.
organize imports, non-functional change.