Allow search results to be snippeted, roughly

Summary:
Ref T8646. This is fairly rough:

This interface is very niche, and not really flexible enough to accommodate other result customization (but I don't think we have any plans here)?

I'm just summarizing the content of documents, basically showing the first paragraph of their content, summary, etc. This isn't what Google does: it shows snippets surrounding the actual search terms. However, this is more involved and might be less useful in structured data: for example, I'd imagine that the first line of most phriciton documents, maniphest tasks and Differential revisions really might be the best machine-generatable summary of them. The actual contextual snippeting in Google doesn't often seem hugely useful to me. But this might also not be very useful.

There's not much design, not sure if you had any ideas.

I only implemented this for tasks, revisions and the wiki since those seem most useful.

I'm generally on the fence about this, but it's not a ton of work to swap out for something else later. Maybe we can see how it feels? But happy to toss it or rethink the approach.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8646

Differential Revision: https://secure.phabricator.com/D14095