Members should not be able to change there username

[dpslwk]

As stated in the title.

Since hms usernames are now used as logins on several other systems (wiki, jarivs, spacenet) and changes made in HMS are not reflected on these external systems and it will be a lot of work to implement change username functions on this growing list of external services.

Daniel and I think we should just disable the edit option on the username field to save a lot of headaches in the future, this also ties in with adding in the Username Black list #45