recent_topics: Set focus on input element on launch.

frontend_tests/node_tests/recent_topics.js

@@ -317,6 +317,7 @@ run_test("test_recent_topics_launch", () => {
     rt.process_messages(messages);
 
     rt.launch();
+    assert.equal($("#recent_topics_search").is(":focus"), true);
     overlays.close_callback();
 
     // incorrect topic_key

static/js/recent_topics.js

@@ -323,6 +323,7 @@ exports.launch = function () {
         },
     });
     recent_topics.complete_rerender();
+    $("#recent_topics_search").focus();
 };
 
 window.recent_topics = exports;