Add route for contribution tasks so they can be used with searchkit

See: #36

The controller class is required to pick up the contribution ids from the request parameter. The tweaks to $this->isSubmitted() checks stop early return when the form loads in a popup (I think because the form is loaded with a POST request to pass the contribution ids?).

To note: this won't work immediately because the SearchKit code for legacy search actions for contributions currently generates a qfKey based on a hard-coded class name (CRM_Contribute_Controller_Task), which only works for core contribution tasks. I have a patch for this which will raise MR for now too: https://github.com/civicrm/civicrm-core/commit/820493f0bae53f25294d98b55b8780034acd6a71 . But I need this MR to demonstrate the need for that patch...