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/bug_report.md

@@ -0,0 +1,37 @@
+---
+name: Bug report
+about: Create a bug report
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+<!-- Please note, this template is for definite bugs, not requests for
+support. If you need help with Tailscale, please email
+support@tailscale.com. We don't provide support via Github issues. -->
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**To Reproduce**
+Steps to reproduce the behavior:
+1. Go to '...'
+2. Click on '....'
+3. Scroll down to '....'
+4. See error
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Screenshots**
+If applicable, add screenshots to help explain your problem.
+
+**Version information:**
+ - Device: [e.g. iPhone X, laptop]
+ - OS: [e.g. Windows, MacOS]
+ - OS version: [e.g. Windows 10, Ubuntu 18.04]
+ - Tailscale version: [e.g. 0.95-0]
+
+**Additional context**
+Add any other context about the problem here.

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: true
+contact_links:
+  - name: Support and Product Questions
+    url: https://tailscale.com/kb/1023/troubleshooting
+    about: Please send support questions and questions about the Tailscale product to support@tailscale.com
+  - name: Security Issues
+    url: https://github.com/tailscale/tailscale/blob/master/SECURITY.md
+    about: You can report security issues privately to security@tailscale.com

.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.