Add hidden text to issue templates

Explains that this is a bug tracker + links to owncloud.org/support

issue_template.md

+<!--
+Thanks for reporting issues back to ownCloud! This is the issue tracker of ownCloud, if you have any support question please check out https://owncloud.org/support
+
+This is the bug tracker for the Server component. Find other components at https://github.com/owncloud/core/blob/master/CONTRIBUTING.md#guidelines
+
+To make it possible for us to help you please fill out below information carefully.
+--> 
 ### Steps to reproduce
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