adding a 'minimal' layout for performace testing.

add the ?layout=minimal parameter to the url to use this.
groups#show: First steps to improve performance.

(1) Remove pagination.  (2) have former members not removed one by one
but using a subtraction.
groups#show: do not display deceased members ...

of a corporation within the corporation overview. This has confused
some people.
groups#show: caching users' titles.
groups#show: do not display the members count.

This causes problems at the moment, since all members, including all
members of the sub groups are counted.

Also, do not list members if there are too many.
groups#show: display `@members` count, not `@group.members`.

The old was removed in this commit:
https://github.com/fiedl/wingolfsplattform/commit/14ffed6cd829ce089efefe
af8eb796b400ef3b54

Trello: https://trello.com/c/RPlYvbcv/645-gruppen-seite-schneller-machen
groups#show: display the number of members for large groups.
issue fixing: make specs green again.

In cases where the objects were `nil` or didn’t respond to the methods,
errors have been raised. This is fixed now.
groups#show for wingolf corporations: list only members of aktivitas and philisterschaft.