From 42e3f6b862cd3c5ad4f25fca7a1fe685a3a1d1e6 Mon Sep 17 00:00:00 2001
From: richvdh
user_id - The fully qualified MXID: for example, @user:server.com. The user must
be local.Checks to see if a username is available, and valid, for the server. See the client-server +API +for more information.
+This endpoint will work even if registration is disabled on the server, unlike
+/_matrix/client/r0/register/available.
The API is:
+POST /_synapse/admin/v1/username_availabile?username=$localpart
+
+The request and response format is the same as the /_matrix/client/r0/register/available API.
+To use it, you will need to authenticate by providing an access_token for a
+server admin: Admin API