BF: Pass desired_user_id to register API. And update the app header to avoid to display the previous matrix id

change the world: make the default matrix API URL prefix /_matrix rather than /matrix to make it easier for existing websites to mount a HS in their namespace without collisions.

perl -pi -e 's#/matrix#/_matrix#g' ./cmdclient/console.py ./docs/client-server/howto.rst ./docs/client-server/specification.rst ./docs/client-server/swagger_matrix/directory ./docs/client-server/swagger_matrix/events ./docs/client-server/swagger_matrix/login ./docs/client-server/swagger_matrix/presence ./docs/client-server/swagger_matrix/profile ./docs/client-server/swagger_matrix/registration ./docs/client-server/swagger_matrix/rooms ./docs/server-server/specification.rst ./graph/graph.py ./jsfiddles/create_room_send_msg/demo.js ./jsfiddles/event_stream/demo.js ./jsfiddles/example_app/demo.js ./jsfiddles/register_login/demo.js ./jsfiddles/room_memberships/demo.js ./synapse/api/urls.py ./tests/federation/test_federation.py ./tests/handlers/test_presence.py ./tests/handlers/test_typing.py ./tests/rest/test_events.py ./tests/rest/test_presence.py ./tests/rest/test_profile.py ./tests/rest/test_rooms.py ./webclient/components/fileUpload/file-upload-service.js ./webclient/components/matrix/matrix-service.js

 - add a simple CSS template across the app for navigation & cosmetics
 - split login into login & register, and totally reskin it
 - restructure room CSS to play nicely with it
 - implement basis 1:1 chat from user pages
 - disable autofocus on iOS to improve UX

 * No need to inform clients of status of remote users; as that will
   arrive in due course anyway. We don't -have- the state currently, so
   we'd only send an unknown message

 * Remember to bump the presence serial for the event source, so the
   notifiers will wake up and report it

Have EventSource's get_new_events_for_user() API work only on keys within that source, not overall eventstream tokens

Avoid hardcoding names of individual stream token keys in its own implementation; this at least reduces the number of places in source code the individual parts are stored