API: Add filtering using field lookups to UnitResource

* Units are only listed when a filter is provided.
* Filter by state allows using human-readable names: fuzzy, translated...
* Also expand UnitResource docs to document this feature.

Fix bug 2982
API: Add UserResource resource and docs

Fix bug 2959
API: Add statistics to UserResource

Fix bug 2960
API: Add UserObjectsOnlyAuthorization

This will allow to forbid access to User resources that belong to other users

Fix bug 2958
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