New page to get admin on existing account via a code logged on server

[IRHM]

Since #141, we need an easy way for existing users to get admin (on existing installs that are upgrading).

We can make a simple page that requests a code from the server, which will be logged to console. Only the server admin should have access to that log, so it should be safe to assume if the user has that code, they are an admin. When typed in, we can update their user permissions to admin.