update issue template to current github format

[maxheld83]

split off from #755 as suggested by @jennybc.

[maxheld83]

maybe this might be a tdd candidate?

[jennybc]

The intent of this issue was to change the issue template used by use_tidy_issue_template(), which #818 PR doesn't do. I.e. we have to change the file inst/templates/tidy-issue.md.

I'm going to reopen.

@Maschette Do you want to take another crack at it? I'm going to point out a few things in the original PR.