[WIP] Focus share link when pop-up opens

Fixes #1673 by waiting for the dropdown to open and then selecting and focusing input.

- [ ] Needs tests.
Okay thanks, the `scope.$evalAsync` works. The problem with using the method on the scope is that we end up re-implementing the dropdown-toggle directive. The above diff currently doesn't deal with closing the popover again[1].

The other alternative I can think of is to just implement our own version of dropdown-toggle that triggers a "toggle" event on the node which we can listen to. Or I just leave this to sniff the class... 

[1] https://github.com/hypothesis/h/blob/1673-focus-share-link/h/static/scripts/vendor/angular-bootstrap.js#L40-51