Removed JS junk
Checks if nav item exists before adding listener

Was getting an annoying Chrome breakpoint when the user was signed out
and thus this button didn't exists
Corrected relative URIs (thanks @DanielKoehler)
Prompt user to sign in if not. Also, stop users from buying own items
Allows re-selecting an image before uploading
Adds "Sign up" nav item. Fixed relative URI.
Checks if user data is null before trying to read

Caused an exception before