Warn users about the consequences of deleting a project

[di]

Fixes #6318.

[brettcannon]

What does "with 1 release" mean? Is the latest release taken out but the files for all other releases left behind? (Basically I'm wondering if project deletion leads to a situation as bad as left-pad or just more of an index deletion than a complete nuking of the project).

[di]

The "with N release(s)` line is already present, it links to all the existing releases (and the number changed depending on the number of releases).

From a user's perspective, project deletion deletes everything: the project, all releases, all files.

Behind the scenes: we delete everything except the actual distribution file in our storage bucket, but the only way a user would be able to continue installing a deleted distribution would be to have the URL to that file and do pip install https://files.pythonhosted.org/../............. We also maintain a reference to what distributions once existed, which allows us to continue preventing filename reuse.

[nlhkabu]

Updated as discussed @di