Rename user accounts

[brollb]

In order to be able to transition to case-insensitive accounts, it would be convenient to be able to have mechanisms for renaming accounts (so we don't have to do it from the database itself). There are a couple complexities:

• it would be good to introduce an ID field and use this when checking against banned accounts
• projects paths on s3 should use the ID rather than the username to avoid unnecessary renames
• collaborators will need to be renamed (or switch to use the ID field though this will impact the client side, potentially)
• project owner fields need to be updated or switched to IDs