Table MM_USER: rename SALT attribute to PASSWORD_SALT
List table MM_MEDIA before MM_USER_AXIS and MM_MEDIA_AXIS
Table MM_MEDIA: remove column USER_NAME (already in MM_USERS)

There is no need to store duplicate information in MM_MEDIA: the ID
is everything to we need to know, as the username is already stored
in the MM_USER table.
Add brief explanation of the SQL tables