Fix User endpoints for self user

[sebastian-echeverria]

Fix handling of endpoints for self user. More specifically:

• Prevent users with "me" username from being created
• Ensure /user/me endpoint works correctly with authorization
• Ensure edit user is possible for current user
• And ensure a user, while editing itself, cannot add groups to itself
• Add /user/me/models endpoint to give all models the current user has access to

[sebastian-echeverria]

• User/me endpoint fixed
• Models for user/me working

[sebastian-echeverria]

Current issue editing user is that id is obtained from URL, but in the edit case, the id comes in the body; thus, the write-user permission is incorrectly checked, instead of the write-[miid]-user

[sebastian-echeverria]

Addressed in #434