Mark entries in the `UserRepository` as `OLD` after some time

[saemy]

Currently, users that have been loaded into UserRepository are never reloaded. For long-living apps, updates to a user's profile on the website are never showing up in the app.

Mark entries in the UserRepository as OLD after some time (how long?) s.t. they get reloaded from the website.

[sgelb]

I would think that most user profiles are pretty static, so perhaps once a day and/or the possibility of pull-to-refresh?