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

spec/components/concern/positionable_spec.rb

@@ -12,11 +12,11 @@ describe Positionable do
         include Positionable
       end
 
-      Topic.exec_sql("create temporary table test_items(id int primary key, position int)")
+      DB.exec("create temporary table test_items(id int primary key, position int)")
     end
 
     after do
-      Topic.exec_sql("drop table test_items")
+      DB.exec("drop table test_items")
 
       # import is making my life hard, we need to nuke this out of orbit
       des = ActiveSupport::DescendantsTracker.class_variable_get :@@direct_descendants
@@ -25,7 +25,7 @@ describe Positionable do
 
     it "can position stuff correctly" do
       5.times do |i|
-        Topic.exec_sql("insert into test_items(id,position) values(#{i}, #{i})")
+        DB.exec("insert into test_items(id,position) values(#{i}, #{i})")
       end
 
       expect(positions).to eq([0, 1, 2, 3, 4])