Add GET /register/available
Add GET /register/available

Adds https://github.com/matrix-org/matrix-doc/issues/911

Signed-off-by: Travis Ralston <f36f6a6abc4b55589c96c71fceedf83e596b8f33@gmail.com>
matrix-org/matrix-doc,matrix-org/matrix-doc,matrix-org/matrix-doc,matrix-org/matrix-doc