.github: add big warning signs about security bugs

.github/ISSUE_TEMPLATE/bug_report.md

@@ -5,10 +5,13 @@ about: Create a report to help us improve
 ---
 
 <!--
+====================================
+IF YOUR ISSUE IS RELATED TO SECURITY
+====================================
+please submit it to the security mailing-list security@riot-os.org.
+
 If your issue is a usage question, please submit it to the user mailing-list
 users@riot-os.org or to the developer mailing-list devel@riot-os.org.
-If your issue is related to security, please submit it to the security
-mailing-list security@riot-os.org.
 -->
 
 #### Description

.github/ISSUE_TEMPLATE/security_bug.md

+---
+title: 'Security Bug report'
+about: |
+    Don't use the issue tracker for this! Please write an e-mail to
+    security@riot-os.org. The button was just added to advertise this message.
+---
+
+# DON'T USE THE ISSUE TRACKER FOR THIS!
+<!--
+Please write an e-mail to security@riot-os.org. The button was just added to
+advertise this message!
+-->