Refactor Document Equivalence

[leedenison]

This master issue collects together issues related to document equivalence:

• https://github.com/hypothesis/product-backlog/issues/903
• https://github.com/hypothesis/product-backlog/issues/1437
• https://github.com/hypothesis/h/issues/6552
• https://github.com/hypothesis/h/issues/6576
• https://github.com/hypothesis/h/issues/6033
• https://github.com/hypothesis/h/issues/6216
• https://github.com/hypothesis/product-backlog/issues/1430
• https://github.com/hypothesis/h/issues/7884
• https://github.com/hypothesis/support-legacy/issues/154
• https://github.com/hypothesis/product-backlog/issues/876
• https://github.com/hypothesis/private-issues/issues/45
• https://github.com/hypothesis/product-backlog/issues/1375
• https://github.com/hypothesis/h/issues/7884
• https://github.com/hypothesis/product-backlog/issues/1369
• https://github.com/hypothesis/product-backlog/issues/1291
• https://github.com/hypothesis/h/issues/5359