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

db/migrate/20140604145431_disable_external_auths_by_default.rb

@@ -1,15 +1,15 @@
 class DisableExternalAuthsByDefault < ActiveRecord::Migration[4.2]
 
   def enable_setting_if_default(name)
-    result = User.exec_sql("SELECT count(*) count FROM site_settings WHERE name = '#{name}'")
-    if result[0]['count'].to_i == 0
+    result = DB.query_single("SELECT count(*) count FROM site_settings WHERE name = '#{name}'")
+    if result.first.to_i == 0
       execute "INSERT INTO site_settings (name, data_type, value, created_at, updated_at) VALUES ('#{name}', 5, 't', now(), now())"
     end
   end
 
   def up
-    users_count_query = User.exec_sql("SELECT count(*) FROM users")
-    if users_count_query[0]['count'].to_i > 1
+    users_count_query = DB.query_single("SELECT count(*) FROM users")
+    if users_count_query.first.to_i > 1
       # existing site, so keep settings as they are
       enable_setting_if_default 'enable_yahoo_logins'
       enable_setting_if_default 'enable_google_oauth2_logins'