!!! THIS IS WIP !!!

This PR has a few commits from #1897, but once it gets merged, only the commits to address the custom assets will be displayed.

I'm opening this PR to get feedbacks from @ewdurbin on the DB modeling. Here's how I though about how to support generic assets under the sponsorship app:

1. Define a pair of benefit feature/configuration models so admin users can configure the benefits;
2. Every feature/configuration must define an unique internal_name and a related_to choice field (sponsors / sponsorship). Both fields will be controlled to know to which object the asset should relate to and also to query over Generic FKs using the internal_name to check if the sponsor and/or sponsorship has the required asset;
3. The dashboard can iterate over all of the sponsorship's benefits and check the ones which require assets. Then, we'll use the internal_name and related_to object to fetch for the stored data in our DB via a more generic model;

I was thinking on following a very similar strategy with the documents assets (docx, pdf, ppts etc). We could define a new RequiredDocumentAsset configuration/benefit following the same strategy and using a new GenericFileAsset class to store the uploaded document.