Allow keyboard users to move from annotation card to anchor text and vice versa

[this is a usability improvement and not part of WCAG compliance work]

Selecting an annotation within the Hypothesis panel to jump to its location in the content and jumping to an annotation in the sidebar from the text lack a keyboard equivalent.

From Kyle: tabIndex="0" on highlights works nicely, not sure I would want that enabled all the time, but for getting back to the highlight in its context, it works well