FEATURE: allow disabling of extra term injection in search

There is a feature in search where we take over from the tokenizer
in postgres and attempt to inject more words into search.

So for example: sam.i.am will inject the words i and am.

This is not ideal cause there are many edge cases and this can
cause extreme index bloat.

This is an opening move commit to make it configurable, over the
next few weeks we will evaluate and decide if we disable this by
default or simply remove.

config/site_settings.yml

@@ -1730,6 +1730,9 @@ backups:
     hidden: true
 
 search:
+  search_inject_extra_terms:
+    default: true
+    hidden: true
   min_search_term_length:
     client: true
     default: 3