Advanced search fixes

[longhotsummer]

• build the search query field-by-field, so that it's correct
• don't mix simple and advanced search for "all" field queries; so advanced search now has search__all; this means we can apply stricter match requirements
• adjust field chooser so that the user can't choose no fields, or all + other fields
• advanced search on the "content" field must search content, pages and provisions

Closes #1795

https://www.loom.com/share/e0e891516fdd488cab852e9ca663c387

There is still a problem with negative queries on pages or legislation with provisions. Because each provision/page is indexed and search individually, it's possible to have a search dog -cat that returns a document with cat in it, because it'll match pages or provisions that have dog but not cat.

So for negative searches we actually need to index and search the full content, not on a per-page or per-provision basis.

[longhotsummer]

I've opened https://github.com/laws-africa/peachjam/issues/1815 for the negative search issue