Manual::Config: add a sample call to attached_image_urls_builder
busybird.css: "pointer" cursor should be set to the "extension-toggler", not the "extension-container" in general.
timeline.js: prevent double-click on extension-container from toggling the pane twice. The "click" event handler is disabled while the pane is toggling.
timeline.js: StatusContainer#toggleExtensionPane(): now it collapses into the line a little above the screen center. This is because the extension pane expands downwards.