search: support quoted phrases, OR, and NOT operators in full-text search.

Make all full-text search fields support quoted phrases and OR and NOT
operators.

This affects all text search fields (any search field that looks like `*_matches`).

Examples:

* hakurei reimu   - matches anything containing the words "hakurei" and "reimu", in any order.
* hakuri or reimu - matches either "hakurei" or "reimu".
* hakurei -reimu  - matches "hakurei" but not "reimu"
* "hakurei reimu" - matches the exact phrase "hakurei reimu"
* "reimu hakurei" - matches the exact phrase "reimu hakurei"

* https://danbooru.donmai.us/notes?search[body_matches]=reimu+hakurei
* https://danbooru.donmai.us/notes?search[body_matches]=reimu+or+hakurei
* https://danbooru.donmai.us/notes?search[body_matches]=reimu+-hakurei
* https://danbooru.donmai.us/notes?search[body_matches]="hakurei+reimu"
* https://danbooru.donmai.us/notes?search[body_matches]="reimu+hakurei"

The phrase search ability partially fixes #4536 (Inconsistent behavior
of search function for comments/forums).

See `websearch_to_tsquery` [1] for full details of the search syntax.

[1]: https://www.postgresql.org/docs/current/textsearch-controls.html#TEXTSEARCH-PARSING-QUERIES

app/logical/concerns/searchable.rb

@@ -167,7 +167,7 @@ module Searchable
       to_tsvector("pg_catalog.english", arel_table[column])
     end.reduce(:concat)
 
-    where("(#{tsvectors.to_sql}) @@ plainto_tsquery('pg_catalog.english', :query)", query: query)
+    where("(#{tsvectors.to_sql}) @@ websearch_to_tsquery('pg_catalog.english', :query)", query: query)
   end
 
   def search_boolean_attribute(attr, params)