Create three grades of user: proof, normal, super

[sujato]

Currently all users have the same rights, allowing them to edit their own translation but no other.

The suggestions UI will require a new kind of user, one who can make suggestions only but not edit text or translation.

In addition, we want a superuser who can edit root text and HTML, etc. Thus we need three grades:

• superuser
  • write anywhere—root, variants, HTML, references—except other people's translation and comments
  • write their own translation and comments
  • write suggestions anywhere
  • read anywhere
• normal user
  • write their own translation and comments
  • write suggestions anywhere
  • read anywhere
• proof user
  • write suggestions anywhere
  • read anywhere

User status should be indicated with the user icon in the header.