text: Title: ES Autocomplete search queries should properly use boosted fields 
  Body: ### Is your proposal related to a problem?
  The fixes to Elasticsearch boosting from #10653 never properly made it into autocomplete queries. This means that if you're using the autocomplete method to search pages, it will often give a higher rank to pages with titles that don't match the search results. I've been able to make these changes in the site that I discovered this issue in, but the existing code is written in such a way that it's a challenge to make small changes to search backends without having to duplicate a lot of base Wagtail code, which I'd really like to not do.
  
  ### Describe the solution you'd like
  Autocomplete queries should also have boosted fields that are copied to using the same method as the base query compiler and then queried with a boost at runtime.
  
  ### Describe alternatives you've considered
  If the Wagtail team doesn't want to fully support this, it would still be appreciated to be able to break out some of the existing methods in ways that make it easier to extend.
  
  ### Additional context
  I know that the Wagtail team is generally critical about using partial matching/autocompletion for normal searches, but this came up because I've developed a relatively large Wagtail instance that uses autocomplete queries for their base site search. The client sells a large amount of products, and these products are often referred to by shortened versions of the page title. For example, 299 is a very common search term on our site, and users (and our client) expect to be able to find all of the products sold on the site in that product line, all of which will have titles like 299E4STD3G. In our cases, it makes sense then to use edgengrams for our main search, as that's one of the primary ways that users are browsing the site. I wouldn't be surprised if other Wagtail instances have similar requirements, so I think this is a reasonable usecase to support. 
  
  ### Working on this
  I've already developed a solution for my site, so I know what places in the code need to be changed. I would likely need to do some additional work (tests, etc) to get it ready for the main repo, but I would be happy to work on it. 
  
  Anyone can contribute to this. View our [contributing guidelines](https://docs.wagtail.org/en/latest/contributing/index.html), add a comment to the issue once you’re ready to start.
  