Remove password fields when logged in with Github

If a user has used Github authentication to create their account, they are
not able to update their password as one does not exist. This resolves the
issue by removing the Change Password form, and also the Password input on
the Edit Profile page.

One possible improvement that could be made in future would be additional
checks to ensure the user is still authenticated with Github, though I've
spent all day on this already so I'll leave that to somebody else to add in
;)