implement institution-specific notifications

this MR implements the option for institution-specific notifications. In the filter form, one can now select specific institutions, as well as whether one want to contact all members or only the principal contact persons, and whether inactive institutions should be ignored.

one can then select various placeholder in the email body (and from now on also in the subject!)

We only take active memberships into account, and we send one mail per membership (i.e. if a member participates in two of the selected institutions, he or she will get two mails, one for each institution).

closes #179 (closed)

ping @csteger