Use fancy HTML-enabled Issue Form Templates for issues

[alice-i-cecile]

Structured forms can give a more polished bug report / feature request experience, with drop down menus, separate text fields and more.

This can be seen in the cargo repo, check out the underlying issue templates to see how it's done.

For more docs, see https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/syntax-for-issue-forms

If we do this, we need to ensure that we never disable the "blank issue" option to ensure there's an appropriate safety valve.