Return userid from check_permission

[Maillol]

What do these changes do?

check_permission now returns the userid instead of None. This avoids an extra authorization lookup for callers.

Are there changes in behavior for the user?

These changes does not break existing logic (only adds a returned value)

Related issue number

Checklist

• [X] I think the code is well written
• [X] Unit tests for the changes exist
• [X] Documentation reflects the changes