[META] Improve core search.

Todo:

• [ ] Easier Facets
• [ ] Integration with views
• [ ] Smarter Defaults
• [ ] Add comprehensive settings on a field level
• [ ] #1143 [DX] Provide a way to alter search results
• [ ] #1320 [UX] Implement N-Gram/Porter-Stemmer/Fuzzy/Partial-Word search in core.
• [ ] #1330 Put all search results on a single page (with facets?)
• [ ] #1337 Improve the way search (re)indexing works.
• [ ] #1338 [UX] Convert search to a view.
• [ ] #3385 [UX] Search settings: Convert the "Search items" section to a table.
• [ ] #3517 [UX] Add content title match as search relevancy factor.
• [ ] #3518 [DX] Remove hook_search_admin() in favor of a form_alter.
• [ ] #3850 Search terms along with nodes
• [ ] #4444 Fix exposed search filters in views

Completed issues: - [x] #4197 [UX] Provide a way to prevent some content from appearing in search index - [x] #4210 [UX] Display the default search setting option only if more than one search modules are active - [x] #1282 [UX] Replace "Node" with "Content" in Search Settings (implemented: https://github.com/backdrop/backdrop/issues/1116) - [x] #1321 [UX] Search: make it clear what the 're-index site' button does - [x] ~~#1329 Users in the search index are not properly re-indexed.~~ - [x] #1336 [UX] Simplify and improve the "Search items" section under admin/config/search/settings