split up PowerLevelEditor into two components

one while editing (PowerLevelEditor) and one
while not editing (PowerLevelSection).

Also save when pressing the button, not when changing the
power dropdown.

Also show the spinner next to the dropdown when saving,
not at the bottom of the component.

src/Roles.js

@@ -30,6 +30,6 @@ export function textualPowerLevel(level, usersDefault) {
     if (LEVEL_ROLE_MAP[level]) {
         return LEVEL_ROLE_MAP[level];
     } else {
-        return _t("Custom %(level)s", {level});
+        return _t("Custom (%(level)s)", {level});
     }
 }