Einrichtungsfilter

There are some Problems with the current Instituts-Filter (#34 (closed))

Right now a user can select between institutes which he has Stud.IP-rights for. Instead the Widget should allow to filter within the institutes that a user is allowed to see in context of his Plugin-role.

A list of those institutes is already used for the display of contracts on that page and should be used for a select-widget (existing Stud.IP Widget) to search within that selection.

The Multi Select is not necessarily needed. (the current widget has usability Problems anyway. For insitites with long names, you cannot click the x to remove the filter)