Search: allow searching by file names

[stsewd]

What's the problem this feature will solve?

With #11646 we will now have a list of files that were changed. I think we can also expose a feature that just allows users to search by a filename (and maybe title), something like a fuzzy search you see in editors.

Describe the solution you'd like

Use the manifest to store also the title of each file or maybe just use the information we already have in ES, so we can show that nicely next to the filename. We will just load the full list of files and titles to the client to do the search from the client, don't think we need to do the search server side, as the list of files shouldn't take too much space.

Alternative solutions

Integrate this into our existing search, and expose an option like "search in filename"/"search in title" etc.. I think I'm liking this option more, the only downside is that results will be server side.

[humitos]

This idea is pretty related to what @ericholscher called "File section indexer" in #11791

[ericholscher]

Yea, it's basically the same concept. I think this is really valuable, and I'd love to get something working with it soon.

There's some additional design discussion here: readthedocs/addons#449 (comment) -- in particular about the UX of having in-memory search results, and then delayed server-side search results, and how to show them both.