Wrap diagnostic output into collapsed <details> block

[MurzNN]

The Issue

We need to create a gist now to paste diagnostic information, that is too many actions for a regular user.

How This PR Solves The Issue

This fix should add the pre-filled text to the diagnostic field.

Manual Testing Instructions

Create a new issue and fill the diagnostic data.

Automated Testing Overview

Related Issue Link(s)

Fixes #4244

Release/Deployment Notes

[rfay]

IIRC this approach doesn't work in bug report template, but please try it in your own repo or fork to make sure.

[MurzNN]

There was a whitespace problem, fixed now, here is the example bug: MurzNN#7
Please check.

[rfay]

Thanks, I think this is great. Minor suggestions you can accept (and preferably test in your fork too). I appreciate it. I think the last time this was attempted it didn't work out.