Prevent bypassing permissions using filters.

It was possible to see customers, projects or activities you have no
permission to see by filtering by a user which has entries for any of
these.

This is now prevented by always including the allowed customers,
projects and activities when querying the database. When a filter is
applied the intersection between the set of all allowed IDs and the set
of the selected IDs is used.
Merge pull request #444 from ServiusHack/respect_permissions_with_filters

Prevent bypassing permissions using filters.