License cleanups
Cleanups
API docs: Cleanups, minor fixes, and some additions
API docs: Include a section about list matching a criteria
API: Add filtering by code to LanguageResource

This allows to use the Pootle API to check if a given language code returns
any match, and thus to test if some language exists.
API: Include a proper backlink in ProjectResource

Necessary for Pootle-Solas integration.

always_return_data indicates that all HTTP methods (except DELETE) should
return a serialized form of the data.
API: Allow superusers to see all UserResource objects

This will keep hiding certain fields for all the UserResources that don't
belong to the user making the query, thus providing a certain level of
security.
API: Allow to GET UserResource lists in specific cases

Do not show any object when performing a GET request unless an exact filter
on 'username' is provided.

This allows to keep a restricted control on what the API UserResource exposes
to the outside, while making it possible for certain users (superusers) to
check if a given user exists using the Pootle API.