Optimize the activemembers admin

Ghost User
2017-10-20T14:13:44+02:00

Closes no issue.

### Short description

This MR makes the activemembers admin a lot faster by doing the following two things:

1. Add `member` to `raw_id_fields` in the inline for committee memberships. Before the entire list of members was copied for every committee membership. This resulted in html pages of several megabytes.
2. Remove duplicate queries (for different ids) by adding `select_related` calls in the right places.