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

Gemfile.lock

@@ -176,6 +176,7 @@ GEM
     mini_portile2 (2.3.0)
     mini_racer (0.1.15)
       libv8 (~> 6.3)
+    mini_sql (0.1.4)
     mini_suffix (0.3.0)
       ffi (~> 1.9)
     minitest (5.11.3)
@@ -240,7 +241,7 @@ GEM
     parallel (1.12.1)
     parser (2.5.1.0)
       ast (~> 2.4.0)
-    pg (0.21.0)
+    pg (1.0.0)
     powerpack (0.1.1)
     progress (3.4.0)
     pry (0.10.4)
@@ -453,6 +454,7 @@ DEPENDENCIES
   message_bus
   mini_mime
   mini_racer
+  mini_sql
   mini_suffix
   minitest
   mocha
@@ -471,7 +473,7 @@ DEPENDENCIES
   omniauth-twitter
   onebox (= 1.8.48)
   openid-redis-store
-  pg (~> 0.21.0)
+  pg
   pry-nav
   pry-rails
   puma