Make Autocomplete more accessible to screen reader users

Use ARIA to:
 + notate that the composer has an autocomplete
 + notate the open/closed state of the autocomplete
 + notate the name of the open autocomplete options
 + notate the ID of the highlighted autocomplete option
 + improve naming of emoji autocomplete options for screen readers

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

src/autocomplete/UserProvider.js

@@ -164,7 +164,7 @@ export default class UserProvider extends AutocompleteProvider {
     }
 
     renderCompletions(completions: [React.Component]): ?React.Component {
-        return <div className="mx_Autocomplete_Completion_container_pill">
+        return <div className="mx_Autocomplete_Completion_container_pill" role="listbox" aria-label={_t("User Autocomplete")}>
             { completions }
         </div>;
     }