Implement a loading state between Register and Welcome

[jessmartin]

The current UX allows the user to click away from the Register screen once they have clicked "Register." They should not be able to do that! Once "Register" is clicked, it can take up to a full second or two for user registration and file system bootstrapping to complete. During that time, the interface needs to be "locked" such that the user can't navigate away. For instance, if there is an error during registration, if the user has closed the modal, they will not see the error message.

Some ideas:

• hide the "x" for the modal once "Register" is clicked
• show a spinner in the middle of the screen and dim the rest of the screen until registration completes