Introduce focus

This changeset implements the *focused annotation* concept, as described in #800.

Very briefly, *focused annotations* are the annotations which currently have an uncollapsed annotation card in the sidebar.

User-centered summary of modified behavior:
 * When you create a new annotation, it will be *focused*. (Which means that it lands in the sidebar uncollapsed, as requested in #1124)
 * When you select some annotations by clicking on a highlight, they will be focused, so the annotation cards will be shown opened.
 * When you select some annotations by clicking on a tab on the heatmap, they will be focused, too.
 * In the document, the highlights belonging to focused annotations are always visible.

For a demo, see   https://focus.dokku.hypothes.is