Convert issue template to .yml format

[RayBB]

Same as LemmyNet/lemmy-ui#1200 but for this repo.

Requested by @Nutomic saying:

One problem we are having is that lots of users are opening github issues and deleting the predefined template with mandatory info. It would be good if someone could convert the issue templates for lemmy and lemmy-ui to .yml format, which allows specifying mandatory fields: https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/syntax-for-issue-forms

I'll work on this later if it isn't done yet.