Multiple selections

This implements multiple selections for annotations, as described in #1118.

Dokku is being deployed at https://multiple2.dokku.hypothes.is/

What is supported: 
 * Selection by highlight click
 * Selection by tab click
 * Adding to selection by CTRL + highlight click 
 * Adding to selection by CTRL + tab click.
 * Removing from selection by CTRL + highlight click 
 * Removing from selection by CTRL + tab click.
 * Doing a CTRL-click will switch to Selection scope, if we are not already in it.

(For Mac, try Meta / Command instead of Ctrl)