This also fixes the is_staff constraint, as memberships
that ended today should not give staff access anymore. This is
a particularly important case, as this is typically what happens
when memberships end 'today' and the object is saved.

This breaks Django2 otherwise

Places like the boards page seem to use the default

This typically resulted in incorrect dates for committee chairs
who had previously been member of the committee as nonchair

This is useful e.g. to set end dates to temporary committees
(such as boards)

The previous version did not take into account that
people could have multiple memberships that did not overlap.

Partially implements #101

This simplification relates to #84, but is necessary
for easier membership migration.

- 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)

They are not well fit in the committee model, and are necessary
when we migrate members.

This is incomplete as events etc are not implemented yet, but should suffice as a decent starting point

Closes #27, but we should track this for new apps