Add issue template

.github/issue_template.md

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+**This repository's issues are reserved for feature requests, bug reports, and other issues with Pa11y Dashboard itself.**
+
+If you need help using Pa11y Dashboard, we recommend using [Stack Overflow](https://stackoverflow.com/questions/tagged/pa11y).
+
+For a bug report, please use the template below. To keep the backlog clean and actionable, issues may be immediately closed if they do not follow one this template.
+
+---
+
+## Expected behaviour
+
+
+## Actual behaviour
+
+
+## Steps to reproduce
+
+
+## Environment
+
+_Please specify the versions of Pa11y Dashboard, MongoDB, and Node.js that can be sued to reproduce this issue._