Closes issue #591, Clarified wording of error when trying to update user to the same organization

[brettp]

Closes Issue #591

Summary

Added additional error USER_ALREADY_IN_ORG that is sent if a PUT request tries to update a user to the org it is already in.

Important Changes

• Added new error to PUT /org/:shortname/user/:username endpoint
• Added tests for update user first name and update user org

Testing

Steps to manually test updated functionality, if possible

• 1) As secretariat, update an existing user and set org_short_name to their current org
• 2) Confirm that a 403 error is returned
• 3) Confirm the JSON error is USER_ALREADY_IN_ORG
• 4) Confirm the JSON error message is similar to The user could not be updated because the user '${username}' already belongs to the '${shortname}' organization.