PR checklist improvements

[Firgrep]

Looking to improve the PR checklist further, but not quite sure how. Ideally I'd want less text (or at least less up in the face) and more focus on anything the PR author writes. Perhaps split the template into various boxes.

Possibly add a script that adds labels based on what files are changed in the PR, e.g. whether they are content files or programming related.

In due course we'll try to shift more of the manual work, such as inserting metadata, over to Prepyrus.