feat(project): update issue templates to include a note about support policy

.github/ISSUE_TEMPLATE/Bug_report.md

@@ -8,7 +8,9 @@ about: Create a bug report
 
 Thanks for reporting a bug for Portainer !
 
-Do you need help or have a question? Come chat with us on Slack http://portainer.io/slack/ or gitter https://gitter.im/portainer/Lobby.
+You can find more information about Portainer support framework policy here: https://www.portainer.io/2019/04/portainer-support-policy/
+
+Do you need help or have a question? Come chat with us on Slack http://portainer.io/slack/.
 
 Before opening a new issue, make sure that we do not have any duplicates
 already open. You can ensure this by searching the issue list for this

.github/ISSUE_TEMPLATE/Custom.md

@@ -6,7 +6,9 @@ about: Ask us a question about Portainer usage or deployment
 
 <!--
 
-Do you need help or have a question? Come chat with us on Slack http://portainer.io/slack/ or gitter https://gitter.im/portainer/Lobby.
+You can find more information about Portainer support framework policy here: https://www.portainer.io/2019/04/portainer-support-policy/
+
+Do you need help or have a question? Come chat with us on Slack http://portainer.io/slack/
 
 Also, be sure to check our FAQ and documentation first: https://portainer.readthedocs.io
 -->

.github/ISSUE_TEMPLATE/Feature_request.md

@@ -8,7 +8,7 @@ about: Suggest a feature/enhancement that should be added in Portainer
 
 Thanks for opening a feature request for Portainer !
 
-Do you need help or have a question? Come chat with us on Slack http://portainer.io/slack/ or gitter https://gitter.im/portainer/Lobby.
+Do you need help or have a question? Come chat with us on Slack http://portainer.io/slack/
 
 Before opening a new issue, make sure that we do not have any duplicates
 already open. You can ensure this by searching the issue list for this