Allow admins to edit user avatars

As an admin, it may be necessary to set a user avatar. This could be to remove an inappropriate one, or to set a standard one our org uses. Ideally, this functionality would be exposed via the REST API so user creation can be further automated.

• Sub feature is to "lock" an avatar and not allow it to be edited by a user.
• Sub feature is to allow editing of multiple user's avatars at once

Current ability to accomplish this happens in one of 2 ways:

1. Use a third party plugin to allow me to SU to a user. Then modify it while logged in as them
2. Change the users password, log in as them, make the change. In the event of an LDAP infrastructure, this means locking them out of multiple systems.