This commit 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.

Prefetch the users in the CommitteeInline

Fixes #196

- Use fields instead of fieldset in admin
- Preserve i18n fields from base in ModelTranslateMeta
- Re-enable usage of name for boards

Squashed commits:
[e05e712] Rename Mentors to Mentorship instead of Mentor
[a694422] Fix PEP
[16ecef4] Add 'mentor achievement' to member profiles
[dd10f23] Fix the string format

Gave an exception when saving the model in the admin
[88717d0] Add admin for the mentor model
[08eb778] Change mentors to mentor (all models are singular)

Closes #11

Includes tests and an authentication backend to check the permissions
granted to committees.