Add issue templates and redirects.

"New issues" will now suggest templates for bugs and
feature requests. Support requests get directed to
support@tailscale.com, and security reports to
security@tailscale.com.

Signed-off-by: David Anderson <dave@natulte.net>

.github/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,26 @@
+---
+name: Feature request
+about: Suggest an idea for this project
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**Is your feature request related to a problem? Please describe.**
+
+A clear and concise description of what the problem is. Ex. I'm always
+frustrated when [...]
+
+**Describe the solution you'd like**
+
+A clear and concise description of what you want to happen.
+
+**Describe alternatives you've considered**
+
+A clear and concise description of any alternative solutions or
+features you've considered.
+
+**Additional context**
+
+Add any other context or screenshots about the feature request here.