interpolating quotes and comments into bouncer's metadata to improve social media sharing behavior of hyp.is links

This issue separates itself from https://github.com/hypothesis/product-backlog/issues/166, which envisions turning annotations into textshots that can be shared on social media. That's desirable and will be great to have.

Here the focus is, rather, on the share cards that Twitter, Facebook, and other services produce when fed bouncer URLs.

Such cards expect (in page metadata) an image, a title and a description. It's easy to interpolate an annotation's quote and comment into bouncer, e.g. by this kind of parameterization:

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Doing that yields, e.g.:

## Twitter


In this case the quote is truncated but we also get to see the comment (which if longer would also be truncated)

## Facebook


In this case the quote is all we get, it’s still truncated, nothing of the comment gets through.

## Rocket Chat


## Slack


The issue names no repository files, tests, or entry points. Start by locating the bouncer URL handler and how its page metadata is generated, then check how quote and comment values could reach the Twitter and Open Graph fields. Done means supported social services show useful quote and comment content in cards without breaking metadata escaping or truncation.

Written by the indexing model from the issue text.