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/comment_migration.rb

@@ -13,10 +13,10 @@ class CommentMigration < ActiveRecord::Migration[4.2]
         comment = column[1]
 
         if column_name == :_table
-          ActiveRecord::Base.exec_sql "COMMENT ON TABLE #{table_name} IS ?", comment
+          DB.exec "COMMENT ON TABLE #{table_name} IS ?", comment
           puts "  COMMENT ON TABLE #{table_name}"
         else
-          ActiveRecord::Base.exec_sql "COMMENT ON COLUMN #{table_name}.#{column_name} IS ?", comment
+          DB.exec "COMMENT ON COLUMN #{table_name}.#{column_name} IS ?", comment
           puts "  COMMENT ON COLUMN #{table_name}.#{column_name}"
         end
       end
@@ -35,10 +35,10 @@ class CommentMigration < ActiveRecord::Migration[4.2]
         comment = column[1]
 
         if column_name == :_table
-          ActiveRecord::Base.exec_sql "COMMENT ON TABLE #{table_name} IS ?", comment
+          DB.exec "COMMENT ON TABLE #{table_name} IS ?", comment
           puts "  COMMENT ON TABLE #{table_name}"
         else
-          ActiveRecord::Base.exec_sql "COMMENT ON COLUMN #{table_name}.#{column_name} IS ?", comment
+          DB.exec "COMMENT ON COLUMN #{table_name}.#{column_name} IS ?", comment
           puts "  COMMENT ON COLUMN #{table_name}.#{column_name}"
         end
       end