Build a better tool for textual attributions (like those required by CC licenses)

[SteelWagstaff]

As an author, I want to quickly & easily create and display attributions acknowledging the contributions of others whose content I have integrated into my own book. Pressbooks supports attributions for images (and a theme option to display these at the end of chapters) (pressbooks/pressbooks#1287). Pressbooks does not currently offer users a straightforward way to provide attribution for text or other non-media elements, however. See #118

Lumen Learning has built and uses a standalone plugin that allows this kind of attribution: https://github.com/lumenlearning/candela-citation. We should do this in core (but better), and allow these Lumen created citations to be converted/made compatible with our method for doing this.
See: https://docs.google.com/document/d/1wi6J6xovZLgudQXSpoAtQWT1f4-7yB0cT0jKPMOBltA/edit# (internal google doc) for more details.