Search: implement stable API

[stsewd]

• Make the search view class based
• Fix bug related to pagination
• Rename highlight to highlights
• Raname docstring to content (domains can be declared outside the code too)
• Cache resolved domains while serializing (this reduced a LOT the number of queries in the dashboard from >200 to ~17)

How is this going to be released without breaking search for cached versions?

Requests with the new-api=true param will get the new api, requests that don't set that param will get the old api.
This is temporary, once we release and browsers fetch the new js (1 or 2 weeks?), we can always default to the new api and delete the old code. Also, having the additional query string will prevent from fetching the results from the cache.

Also, I we can hold off on the docs till we make the change if we want.

Closes #5821
Closes #6341

todo

• update the search extension

[ericholscher]

This looks great. I realize a lot of my comments are on code that was already there, but I think trying to clean this up where possible to make it a bit nicer to read is useful. I don't want to do a lot more refactoring, but at least cleaning up some of the magic functional logic and data structures would be 💯

[stsewd]

I have refactored some bits and used another endpoint for the new search response (we will remove/stop supporting the old endpoint, right?). I want to test this a little more next week in case the refactor broke something.

[stsewd]

Also, reminder for myself to update the search extension too https://github.com/readthedocs/readthedocs-sphinx-search/pull/55/files

[stsewd]

Ok, nothing broke, this is ready for review again.

[stsewd]

I'm leaving this warning since the response will change when we start returning relative paths

[ericholscher]

I wonder if we should do the relative path work in another PR after merging this one, before shipping it? Seems nice to get it all done together if possible.

[stsewd]

Sure, I can start working in another PR

[ericholscher]

These changes look good -- I think this will be a good step forward 🎉