Focus share link when pop-up opens

Fixes #1673 by waiting for the dropdown to open and then selecting and focusing input.
@tilgovi could do with your eyes here, the focus is not triggered here until you mouse out side of the annotation card. I wonder if this is something to do with the dropdown-toggle directive not triggering a digest? It just uses `addClass`[1]. I've tried calling `$digest` manually but that doesn't seem to help either.

Secondly, I'm checking the dom state in the watcher which feels wrong, but I can't thing of another way of checking the dropdown state. I guess I could add a second handler on the toggle button, but that feels counter intuitive.

[1] https://github.com/hypothesis/h/blob/1673-focus-share-link/h/static/scripts/vendor/angular-bootstrap.js#L38