chore(github): complete issue/PR templates and wire in-app bug report to them

Add a feature request template and a config.yml (blank issues disabled,
security reports routed to SECURITY.md) alongside the existing bug report
template, plus a PR template with a checklist matching the project's
security rules. Point the app's "Report a bug" button at the new GitHub
template now that the repo is hosted there instead of the old self-hosted
tracker.

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,5 @@
+blank_issues_enabled: false
+contact_links:
+  - name: Security vulnerability
+    url: https://github.com/davide3011/PalladiumWallet/security/policy
+    about: Please report security vulnerabilities privately, not as a public issue. See SECURITY.md.