add option to let user account start and expire at a given date / datetime

Description

• given you work in projects for short periods in time, it would be nice to have the ability to define a end of usage per user.

Proposal

• add 2 new fields start and enddate into user account

• this may also make it easier for small companies to understand their flow of users through gitlab

• (f.e. we have 40 users inside, where 5 are constantly working, and some of the 40 have never or very seldom used gitlab)