Disallow account creation via openid

[rgrp]

Superticket: #1343

Creation of accounts from OpenID causes large number of problems:

• No guarantee of other profile info
• Poor username (just from openid)

We therefore will:

1. Require creation of user account via register (no auto-creation of accounts via openid)
   • sidebar of register page should point to login page rather than openid signin

2. Permit association of an openid with an account ...
   • How: paste in your openid url (requires change to edit form to have this)
     • won't work for google with their weird openid urls
   • [future] Nicer way is to have login via openid while logged in (which association then happening in background)

3. Migration (for 1+2):
   • (?) Generate decent usernames for all existing accounts
   • As this is nontrivial suggest instead we allow editing of usernames (by account owner and sysadmins). This is useful in its own right and is a reasonable 80/20 solution.

4. [optional] Change display_name to always be username
   • Change My Account to Username at top right of all pages (once we know usernames are short ...)
     • could do this straight away by truncating long usernames (e.g. truncate at 20 chars ...)
     • Also should we lose the icon?