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/group_user.rb

@@ -25,7 +25,7 @@ class GroupUser < ActiveRecord::Base
 
   def set_primary_group
     if group.primary_group
-      self.class.exec_sql("
+      DB.exec("
         UPDATE users
         SET primary_group_id = :id
         WHERE id = :user_id",
@@ -35,7 +35,7 @@ class GroupUser < ActiveRecord::Base
   end
 
   def remove_primary_group
-    self.class.exec_sql("
+    DB.exec("
       UPDATE users
       SET primary_group_id = NULL
       WHERE id = :user_id AND primary_group_id = :id",
@@ -45,7 +45,7 @@ class GroupUser < ActiveRecord::Base
 
   def remove_title
     if group.title.present?
-      self.class.exec_sql("
+      DB.exec("
         UPDATE users SET title = NULL
         WHERE title = :title AND id = :id",
         id: user_id, title: group.title
@@ -55,7 +55,7 @@ class GroupUser < ActiveRecord::Base
 
   def update_title
     if group.title.present?
-      self.class.exec_sql("
+      DB.exec("
         UPDATE users SET title = :title
         WHERE (title IS NULL OR title = '') AND id = :id",
         id: user_id, title: group.title