Don't show register button when registered

[oadaeh]

When a user is already registered, don't show the "Register for Backdrop LIVE" button.

A better thing might be to show a thank you message or a donate message (but not if they have already donated, for that a thank you message and/or an up your donation would be better) or a view the schedule message.

[izmeez]

I made a comment on another issue that any fit better here. Toggle from a "Register" button to a "You are Registered".

If it's possible. It would be great to have a button or block on the page/side-bar present every time you are logged in that says something like, "You Are Registered". Maybe even with an extra note, possibly: "click to select sessions" or something like that.