feature: much more robust location specification

[blahah]

We should use the model PeerLibrary has developed, in which they store essentially all the information available about the location of an annotation

Defined here: https://github.com/peerlibrary/peerlibrary/blob/1e27cc32ee93d4322afce356738c45f24205a729/client/lib/annotator/domanchors.coffee#L31-L50