Normalise case for metadata associated to an annotation

[esanzgar]

The client normalises the case of the metadata associated to an annotation's document. See https://github.com/hypothesis/client/pull/4203

h should do the same thing:

• [ ] store a lowercase normalised version of document metadata relationship
• [ ] do a case insensitive comparison if used for document equivalence
• [ ] remediate already-captured values based on this study: https://github.com/hypothesis/client/pull/4203#issuecomment-1038796042