Ensure settings for Service and Mailer are read on the install page (#15943)

* Ensure settings for Service and Mailer are read on the install page

NewContext does not set the mailer or service settings so add
a new function that will run this.

Fix #15894

Signed-off-by: Andrew Thornton <art27@cantab.net>

* placate lint

Signed-off-by: Andrew Thornton <art27@cantab.net>

routers/install/setting.go

@@ -28,6 +28,7 @@ func PreloadSettings(ctx context.Context) bool {
 			log.Info("SQLite3 Supported")
 		}
 		setting.InitDBConfig()
+		setting.NewServicesForInstall()
 		svg.Init()
 	}