PR for Unattached annotations (=comments) V2

This is an updated version of https://github.com/hypothesis/h/pull/652 by @gergely-ujvari.
(It has been rebased to develop, after merging in the CSS fixes, the username links, and a few other things.)

Demo at [h5](https://h5.nolmecolindor.com/).

Original description follows:

  * * *

This PR allows to create/edit/delete/view annotations without targets (called comments).
These comments do not have quotes (since they do no have targets) and they are collected in a separate tab on the bottom of page.

In this V1 version this separate tab is part of the heatmap, as a separate bucket, which is always shown in the bottom, has custom svg background, and will not be merged with other buckets.
(In V2 it may have its own comment-control-widget)

In the UI these comments has a fake (technical) highlight and range so that our annotation setup does not throw them away, and they're separated from orphaned annotations.

It'll solve #115