Improve delete release warning (closes #4737)

[Cheukting]

Adding warning when user try to delete a release " You will not be able to re-upload a new distribution of the same type with the same version number." also suggest "Consider making a new release or a post release instead."

closes #4737

[di]

Thanks @Cheukting! I don't think testing is needed here, since this is just a change to the HTML template. Let me know when you're ready for a review.

[Cheukting]

@di thanks for your comment. Yes I think it is simple enough. I was thinking I have to run the image and see how it would look like before having it reviewed. However I am encountering problem with running the image locally on my laptop. Wouldn't mind if you or your team review it now and give some comments. I would try to get some help later this month when I join the Sprint in London to make it work eventually.

[di]

Can we link to https://www.python.org/dev/peps/pep-0440/#post-releases on "post release"?

[shevelevs]

I think this PR doesn't actually address the issue. Instead it changes the message that you see on the release page, and only when there are no files. It should be done in a different template (templates/manage_base.html)

[shevelevs]

Here's what the result of this change looks like:

[Cheukting]

Thanks @shevelves, will fix it tomorrow

[di]

@shevelevs is correct, I think this should be in both places (in the pop-up modal, and in the warning box where it currently is).

[shevelevs]

Just to make it clear, this change still shows the improved warning on the main release page only when release has no associated files

[di]

Thanks @Cheukting!