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/notification.rb

@@ -19,10 +19,10 @@ class Notification < ActiveRecord::Base
   after_commit :refresh_notification_count, on: [:create, :update, :destroy]
 
   def self.ensure_consistency!
-    Notification.exec_sql <<-SQL
+    DB.exec(<<~SQL, Notification.types[:private_message])
       DELETE
         FROM notifications n
-       WHERE notification_type = #{Notification.types[:private_message]}
+       WHERE notification_type = ?
          AND NOT EXISTS (
             SELECT 1
               FROM posts p
@@ -152,17 +152,15 @@ class Notification < ActiveRecord::Base
 
     if notifications.present?
 
-      ids = Notification.exec_sql("
+      ids = DB.query_single(<<~SQL, count.to_i)
          SELECT n.id FROM notifications n
          WHERE
            n.notification_type = 6 AND
            n.user_id = #{user.id.to_i} AND
            NOT read
         ORDER BY n.id ASC
-        LIMIT #{count.to_i}
-      ").values.map do |x, _|
-        x.to_i
-      end
+        LIMIT ?
+      SQL
 
       if ids.length > 0
         notifications += user