DEV: remove exec_sql and replace with mini_sql

Introduce new patterns for direct sql that are safe and fast.

MiniSql is not prone to memory bloat that can happen with direct PG usage.
It also has an extremely fast materializer and very a convenient API

- DB.exec(sql, *params) => runs sql returns row count
- DB.query(sql, *params) => runs sql returns usable objects (not a hash)
- DB.query_hash(sql, *params) => runs sql returns an array of hashes
- DB.query_single(sql, *params) => runs sql and returns a flat one dimensional array
- DB.build(sql) => returns a sql builder

See more at: https://github.com/discourse/mini_sql

app/models/tag.rb

@@ -25,7 +25,7 @@ class Tag < ActiveRecord::Base
   end
 
   def self.update_topic_counts
-    Tag.exec_sql <<~SQL
+    DB.exec <<~SQL
       UPDATE tags t
          SET topic_count = x.topic_count
         FROM (
@@ -41,7 +41,7 @@ class Tag < ActiveRecord::Base
          AND x.topic_count <> t.topic_count
     SQL
 
-    Tag.exec_sql <<~SQL
+    DB.exec <<~SQL
       UPDATE tags t
          SET pm_topic_count = x.pm_topic_count
         FROM (
@@ -70,7 +70,7 @@ class Tag < ActiveRecord::Base
 
     filter_sql = guardian&.is_staff? ? '' : " AND tags.id NOT IN (#{DiscourseTagging.hidden_tags_query.select(:id).to_sql})"
 
-    tag_names_with_counts = Tag.exec_sql <<~SQL
+    tag_names_with_counts = DB.query <<~SQL
       SELECT tags.name as tag_name, SUM(stats.topic_count) AS sum_topic_count
         FROM category_tag_stats stats
         JOIN tags ON stats.tag_id = tags.id AND stats.topic_count > 0
@@ -81,7 +81,7 @@ class Tag < ActiveRecord::Base
        LIMIT #{limit}
     SQL
 
-    tag_names_with_counts.map { |row| row['tag_name'] }
+    tag_names_with_counts.map { |row| row.tag_name }
   end
 
   def self.pm_tags(limit_arg: nil, guardian: nil, allowed_user: nil)
@@ -89,8 +89,8 @@ class Tag < ActiveRecord::Base
     limit = limit_arg || SiteSetting.max_tags_in_filter_list
     user_id = allowed_user.id
 
-    tag_names_with_counts = Tag.exec_sql <<~SQL
-      SELECT tags.name, COUNT(topics.id) AS topic_count
+    DB.query_hash(<<~SQL).map!(&:symbolize_keys!)
+      SELECT tags.name as id, tags.name as text, COUNT(topics.id) AS count
         FROM tags
         JOIN topic_tags ON tags.id = topic_tags.tag_id
         JOIN topics ON topics.id = topic_tags.topic_id
@@ -109,8 +109,6 @@ class Tag < ActiveRecord::Base
        GROUP BY tags.name
        LIMIT #{limit}
     SQL
-
-    tag_names_with_counts.map { |t| { id: t['name'], text: t['name'], count: t['topic_count'] } }
   end
 
   def self.include_tags?