Mostly reuse the existing issue template, but with separate template for bug
reports and feature requests.
I made some minor tweaks to the presentation which should hopefully reduce
the propensity of users to side-step the template rather than fill it in. In
particular, most users should usually not need to remove any parts.
I removed specific mentions of Mbed OS since the Mbed TLS project is no
longer tightly associated with Mbed OS.
I removed "Peer device TLS stack and version" because that's rarely relevant
and, in my experience, is mentioned when relevant.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
