Upgrade matrix-registration (v0.7.1 -> 0.7.2) and use official image

This switches us to a container image maintained by the
matrix-registration developer.

0.7.2 also supports a `base_url` configuration option we can use to
make it easier to reverse-proxy at a different base URL.

We still keep some workarounds, because of this issue:
https://github.com/ZerataX/matrix-registration/issues/47

roles/matrix-registration/templates/config.yaml.j2

@@ -28,3 +28,4 @@ logging:
 # password requirements
 password:
   min_length: 8
+base_url: {{ matrix_registration_base_url|to_json }}