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

lib/tasks/posts.rake

@@ -304,7 +304,7 @@ task 'posts:reorder_posts', [:topic_id] => [:environment] do |_, args|
     builder.where("topic_id = :topic_id") if args[:topic_id]
     builder.exec(topic_id: args[:topic_id])
 
-    Notification.exec_sql(<<~SQL)
+    DB.exec(<<~SQL)
       UPDATE notifications AS x
       SET post_number = p.sort_order
       FROM posts AS p
@@ -313,7 +313,7 @@ task 'posts:reorder_posts', [:topic_id] => [:environment] do |_, args|
             p.post_number < 0
     SQL
 
-    PostTiming.exec_sql(<<~SQL)
+    DB.exec(<<~SQL)
       UPDATE post_timings AS x
       SET post_number = x.post_number * -1
       FROM posts AS p
@@ -329,7 +329,7 @@ task 'posts:reorder_posts', [:topic_id] => [:environment] do |_, args|
             p.post_number < 0;
     SQL
 
-    Post.exec_sql(<<~SQL)
+    DB.exec(<<~SQL)
       UPDATE posts AS x
       SET reply_to_post_number = p.sort_order
       FROM posts AS p
@@ -338,7 +338,7 @@ task 'posts:reorder_posts', [:topic_id] => [:environment] do |_, args|
             p.post_number < 0;
     SQL
 
-    TopicUser.exec_sql(<<~SQL)
+    DB.exec(<<~SQL)
       UPDATE topic_users AS x
         SET last_read_post_number = p.sort_order
       FROM posts AS p
@@ -362,7 +362,7 @@ task 'posts:reorder_posts', [:topic_id] => [:environment] do |_, args|
     SQL
 
     # finally update the post_number
-    Post.exec_sql(<<~SQL)
+    DB.exec(<<~SQL)
       UPDATE posts
       SET post_number = sort_order
       WHERE post_number < 0