Follow new role scheme in Prosody

Prosody changed its role scheme to only support a single primary role
for each user. In addition, the names of the built-in roles have been
changed. We thus follow those changes to be compatible with the most
recent trunk.

One open question is whether we should switch admin -> operator here,
too (operator being a server-wide admin), but so far there's no need
to.

snikket_web/prosodyclient.py

@@ -885,7 +885,7 @@ class ProsodyClient:
             localpart: str,
             *,
             display_name: typing.Optional[str],
-            roles: typing.Optional[typing.Collection[str]],
+            role: typing.Optional[str],
             session: aiohttp.ClientSession,
             ) -> None:
         payload: typing.Dict[str, typing.Any] = {
@@ -893,8 +893,8 @@ class ProsodyClient:
         }
         if display_name is not None:
             payload["display_name"] = display_name
-        if roles is not None:
-            payload["roles"] = list(roles)
+        if role is not None:
+            payload["role"] = role
 
         async with session.put(
                 self._admin_v1_endpoint("/users/{}".format(localpart)),