Extract info_overlay.js.

There are several ways we open help for keyboard shortcuts,
markdown help, and search operators.

    - from the gear menu
    - from the compose box
    - from the search box
    - hitting ? for keyboard help
    - arrowing/clicking through the tabs

This just moves the relevant code into a module and changes a
bunch of one-line calls in various places.

static/js/ui_init.js

@@ -281,6 +281,7 @@ $(function () {
     drafts.initialize();
     sent_messages.initialize();
     hotspots.initialize();
+    info_overlay.initialize();
     ui.initialize();
     panels.initialize();
     typing.initialize();