Fix tag search order and not to use tsvector (#3611)

* Sort results by the name
* Switch search method to simple `LIKE` matching instead of tsvector/tsquery

Previously we used scores from ts_rank_cd() to sort results, but it didn't work
because the function returns same score for all results. It's not for calculate
similarity of single words. Sometimes this bug even push out exact matching tag
from results.

Additionally, PostgreSQL supports prefix searching with standard btree index.
Using it offers simpler code, but also less index size and some speed.

app/models/tag.rb

@@ -21,22 +21,9 @@ class Tag < ApplicationRecord
   end
 
   class << self
-    def search_for(terms, limit = 5)
-      terms      = Arel.sql(connection.quote(terms.gsub(/['?\\:]/, ' ')))
-      textsearch = 'to_tsvector(\'simple\', tags.name)'
-      query      = 'to_tsquery(\'simple\', \'\'\' \' || ' + terms + ' || \' \'\'\' || \':*\')'
-
-      sql = <<-SQL.squish
-        SELECT
-          tags.*,
-          ts_rank_cd(#{textsearch}, #{query}) AS rank
-        FROM tags
-        WHERE #{query} @@ #{textsearch}
-        ORDER BY rank DESC
-        LIMIT ?
-      SQL
-
-      Tag.find_by_sql([sql, limit])
+    def search_for(term, limit = 5)
+      pattern = sanitize_sql_like(term) + '%'
+      Tag.where('name like ?', pattern).order(:name).limit(limit)
     end
   end
 end