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/bug_report.yml

@@ -28,17 +28,6 @@ body:
     validations:
       required: true
 
-  - type: dropdown
-    id: network
-    attributes:
-      label: Network
-      options:
-        - Mainnet
-        - Testnet
-        - Regtest
-    validations:
-      required: true
-
   - type: textarea
     id: description
     attributes: