Use the new register-specific request token endpoint (https://github.com/matrix-org/matrix-js-sdk/pull/147) and catch the error that it gives if the email is already in use. Also add initial values to the registration form so we can reload it after the error without all the values disappearing, and split out the guest username parameter which was previously called defaultUsername.
1) Correct fix for Velociraptor (we need to find the DOM node and pass that in)
2) Do the same leak fix for the read marker
3) Update the dependency to our fork which is fixed to make the call we do to release memory actually work.
4) Remove the velocity-ui-pack dependency which is unnecessary because velocity-ui is included in the velocity package
