Link quotes back to their original sources

Copying this from https://github.com/hypothesis/vision/issues/6, which was originally copied from https://github.com/hypothesis/h/issues/773:

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Consider this.

An important politician gives a talk, and on the next day, quotes from it are all over the press. It all sounds very compelling, except that one of the most important claims (widely quoted by the media) is not true. (But that is not generally known.)

Let's assume that one of our users knows something that could totally demolish the lie, and so when reading the news article about the interview, he dutifully annotates it, and adds it's refutation. That's all good, but there were 1000 articles about the talk around the globe, and he has added the corrective information to only one of them.

Obviously, he can't be expected to annotate all occurrences of the quote with the corrective information.

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Situations like this could be solved if we had a way to link quotes in documents to their sources, and thus united the discussion space

It could work like this:
1. The user reads the news article. (And gets pissed.)
2. User looks up the URL of the source (for example, the official press release, with the full transcription of the text.)
3. User highlights the quote in the article, and after activating the dedicated function (on the highlight), he enters the URL of the source, telling the system that this is a quote from that source.
4. The system them tries to search for the text of the current highlight on the source page. If it matches, then it creates a highlight of that part, and registers the connection. After this point, any annotation happening on the source text (inside the quoted section) should appear on all the quotes that are linked back to the source, and vica versa.
5. The user can proceed to submit reply to the highlight (either at the source document, or at the particular news article; it does not matter any more). Or he can annotate something inside the quote.
6. As the user continues to go through the morning papers, he might encounter several other instances of the same quote. All he has to do now is to link them back to their source; with this, his refutation will automatically appear everywhere. (Together with the whole ensuing discussion.)
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I think this is a natural next step, after cross-format annotations. After transcending the boundaries of file formats, we should also transcend the boundaries of documents, so that information is able to flow freely and transparently between all articles quoting the same statements.

The issue describes linking quoted highlights to source documents and sharing annotations across linked passages, following cross-format annotations. No repository files, tests, or entry points are named, so begin by locating the existing highlight, annotation, and document-linking concepts before defining the design and validation criteria.

Written by the indexing model from the issue text.