compose: Return focus to DM input after pill selection.

Before this change, the cursor left the DM recipient box after
selecting a pill from the typeahead. This was only the case with
mouse clicks and not with keyboard selection.

This change ensures we always return focus to the input field
after selection.

web/src/compose_pm_pill.js

@@ -29,6 +29,7 @@ export function initialize({on_pill_create_or_remove}) {
 
     widget.onPillCreate(() => {
         on_pill_create_or_remove();
+        $("#private_message_recipient").trigger("focus");
     });
 
     widget.onPillRemove(() => {