Separate issue templates for bugs and enhancements

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>

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,35 @@
+---
+name: Bug report
+about: To report a bug, please fill this form.
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+### Summary
+
+
+
+### System information
+
+Mbed TLS version (number or commit id): 
+Operating system and version: 
+Configuration (if not default, please attach `config.h`): 
+Compiler and options (if you used a pre-built binary, please indicate how you obtained it): 
+Additional environment information: 
+
+### Expected behavior
+
+
+
+### Actual behavior
+
+
+
+### Steps to reproduce
+
+
+
+### Additional information
+

.github/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,17 @@
+---
+name: Enhancement request
+about: To request an enhancement, please fill this form.
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+### Suggested enhancement
+
+
+
+### Justification
+
+Mbed TLS needs this because 
+