FEATURE: Roll out new search optimisations (#20364)

- Reduce duplication of terms in post index from unlimited to 6. This will
result in reduced index size and reduced weighting for posts containing
a huge amount of duplicate terms. (Eg: a post containing "sam sam sam sam
sam sam sam sam", will index as "sam sam sam sam sam sam", only including
the word up to 6 times.) This corrects a flaw where title weighting could
be ignored.

- Prioritize exact matches of words in titles. Our search always performs
a prefix match. However we want to give special weight to exact title matches
meaning that a search for "sum" will find topics such as "the sum of us" vs
"summer in spring".

- Pick up fixes to our search algorithm which are missing from old indexes.
Specifically pick up the fix that indexes URLs properly. (`https://happy.com`
was stemmed to `happi` in keywords and then was not searchable)

see also:

https://meta.discourse.org/t/refinements-to-search-being-tested-on-meta/254158

Indexing will take a while and work in batches, in the background.

spec/lib/search_spec.rb

@@ -122,6 +122,8 @@ RSpec.describe Search do
 
       before do
         SearchIndexer.enable
+        SiteSetting.max_duplicate_search_index_terms = -1
+        SiteSetting.prioritize_exact_search_title_match = false
         [post1, post2].each { |post| SearchIndexer.index(post, force: true) }
       end