github: Switch to yaml issue templates

These provide more flexibility and can be designed to extract more
information from users when submitting issues.

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,8 @@
+blank_issues_enabled: true
+contact_links:
+  - name: Bitcoin Core Security Policy
+    url: https://github.com/bitcoin/bitcoin/blob/master/SECURITY.md
+    about: View security policy
+  - name: Bitcoin Core Developers
+    url: https://bitcoincore.org
+    about: Bitcoin Core homepage