Ask for server URL before showing sign in box

Contact the server's .well-known API before anything to get
available login flows instead of blindly assuming it will be
m.login.password, and to get the server's real URL instead of
requiring users to remember that e.g. it's "chat.privacytools.io"
and not just "privacytools.io" despite user IDs making it look like so.

The server field will also now remember the last accepted URL.

src/backend/matrix_client.py

@@ -152,11 +152,13 @@ class MatrixClient(nio.AsyncClient):
     }
 
 
-    def __init__(self,
-                 backend,
-                 user:       str,
-                 homeserver: str           = "https://matrix.org",
-                 device_id:  Optional[str] = None) -> None:
+    def __init__(
+        self,
+         backend,
+         user:       str           = "",
+         homeserver: str           = "https://matrix.org",
+         device_id:  Optional[str] = None,
+    ) -> None:
 
         if not urlparse(homeserver).scheme:
             raise ValueError(