Allow to extend El Proxito views from commercial

[humitos]

This is useful in commercial site to override allowed_user method to the required auth checks and decide if the file should be served to the user or not.

In the community site, this method should always return True since all projects/versions are public.

This is all the code changes required in the community site. Feel free to suggest a better pattern or better names for the methods.

This PR is based on #6396

[ericholscher]

I was hoping to have cleaner logic around this, so this makes sense to me as a starting point 👍. It would be cool if it was a mixin that was used elsewhere in our views, so we only have the logic in one place, but we can work towards that.

[humitos]

Yes. I'm not super happy with this design. Although, I didn't come up with a better one. As you said, the can find out a better solution later while we start testing/developing what we need in commercial. The change is not too big even if we want to change the direction later.