Cache user retrieval

The user_id is stored in the flask session. On every page load, we query
the database to get:
- the user objet (user_account table)
- the user's groups (group table) to know what the user can access

If we try to cache the user_load function, the following error is raised
when retrieving the groups:
sqlalchemy.orm.exc.DetachedInstanceError: Parent instance <User at
0x7f51e050a940> is not bound to a Session; lazy load operation of
attribute 'grp' cannot proceed

The groups from AD are only stored to map them with CSENTRY_LDAP_GROUPS.
We don't do any query on them (get all users from one group).
As we always query them with one user, it's more efficient to store them
in the user_account table as an array than in a separate table.
This allows to easily cache the load_user function.

Note that we have to remember to add the current_user to the sqlalchemy session if
we want to modify it or access a relationship (like tokens)!

The load_user function is now cached for 30 minutes. This decreases a
lot the number of database queries.

Fix INFRA-156