status.tx: Add "Expand" and "Collapse" strings to the toggle button for extension pane.
busybird.css: slightly increase the gap between the status text and the extension pane.
status.tx, busybird.css: improve visibility of the extension handle.
busybird.css: add padding to "hidden statuses" header elem.
status.tx, timeline.js: add link to the attached images.
busybird.css: improve recognizability that the image is linked.
Main::PSGI::View: rename "bb_media_image_urls" template function to "bb_attached_image_urls".