Pass custom settings to the DB's SESSION (#1063)

- allows queries to refer to current_setting('app.settings.foo') to retrieve variables
- useful for 12-factor apps (app data can be in environment)
- provides workaround for AWS Relational Database Service (RDS) not
  allowing `ALTER DATABASE SET 'app.[KEY]' TO '[VALUE]'` on database.

test/SpecHelper.hs

@@ -73,6 +73,9 @@ _baseCfg =  -- Connection Settings
             10 Nothing (Just "test.switch_role")
             -- Debug Settings
             True
+            [ ("app.settings.app_host", "localhost")
+            , ("app.settings.external_api_secret", "0123456789abcdef")
+            ]
 
 testCfg :: Text -> AppConfig
 testCfg testDbConn = _baseCfg { configDatabase = testDbConn }