Make footnote anchors unchanging

Problem/Motivation

Sometimes users will share a link to a footnote anchor without being aware that it is radomly generated. This gives rise to broken links. Or users will want to share a link to a footnote and be frustrated becasue they cannot.

Proposed resolution

1. Allow manual entry of and ID and footnote and use it where available. This feature would also be useful in v.3.
2. Create a data-entity-uuid, as CKEditor does for image uploads, and use an ID derived from that as the footnote ID.