Make it clearer that #synapse:matrix.org is our support channel (#7379)

This PR moves the "support is in #synapse:matrix.org" in the bug report template outside of the comment as some people seem to ignore what's in the comments, and phrase it a bit more like the support request template. It also adds a default issue template that says the same thing. It's also adding a notice about the security disclosure to both the default template and the bug report one.

It also adds a badge to the top of the README with an alt text saying about the same message if the badge doesn't load (e.g. if matrix.org is slow).

Fixes #6826 

.github/ISSUE_TEMPLATE.md

+**If you are looking for support** please ask in **#synapse:matrix.org**
+(using a matrix.org account if necessary). We do not use GitHub issues for
+support.
+
+**If you want to report a security issue** please see https://matrix.org/security-disclosure-policy/

.github/ISSUE_TEMPLATE/BUG_REPORT.md

@@ -4,11 +4,13 @@ about: Create a report to help us improve
 
 ---
 
-<!--
+**THIS IS NOT A SUPPORT CHANNEL!**
+**IF YOU HAVE SUPPORT QUESTIONS ABOUT RUNNING OR CONFIGURING YOUR OWN HOME SERVER**,
+please ask in **#synapse:matrix.org** (using a matrix.org account if necessary)
 
-**IF YOU HAVE SUPPORT QUESTIONS ABOUT RUNNING OR CONFIGURING YOUR OWN HOME SERVER**:
-You will likely get better support more quickly if you ask in ** #synapse:matrix.org ** ;)
+<!--
 
+If you want to report a security issue, please see https://matrix.org/security-disclosure-policy/
 
 This is a bug report template. By following the instructions below and
 filling out the sections with your information, you will help the us to get all

README.rst

+================
+Synapse |shield|
+================
+
+.. |shield| image:: https://img.shields.io/matrix/synapse:matrix.org?label=support&logo=matrix
+  :alt: (get support on #synapse:matrix.org)
+  :target: https://matrix.to/#/#synapse:matrix.org
+
 .. contents::
 
 Introduction

changelog.d/7377.doc

+Spell it in an even more obvious way that the support channel for Synapse is `#synapse:matrix.org`.