Re-implement focusing annotation cards

... when hovering over the highlights in the document,
or over the annotation cards themselves.
Don't focus on replies in hover
Use focus outline instead of the drop shadows
Focusing: identify annotations by tag, instead of ID

... because new and deleted annotations don't have IDs,
only tags, assigned by the bridge.
Simplify annotation card focusing logic

Don't bother looking up the annotations (for the tags),
since we only need the list of tags anyway.

(It was different when we worked with the IDs, but we
moved to use tags instead, so the actual annotations
are no longer needed.)
Change annotation card focusing implementation

So that
 - Instead of a map, we now store a list of focused tags
 - The check for being focused has been moved
   into a method in the controller
 - The sidebar now exepts simply a list of tags to
   focus on (no more add/remove tags to existing list)
Use js-hover instead of card-emphasis
Merge pull request #1729 from hypothesis/1728-drop-shadows

Re-implement hiliting annotation cards on hover