Update the GitHub Issue and PR templates to a new format and add a CLA workflow

.github/contributing.md

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+## Contributing
+
+#### Feature Requests
+
+Any feature requests should be posted to the [Feature
+Requests](https://forum.juce.com/c/feature-requests/) section of the JUCE
+forum.
+
+#### Bug Reports
+
+The [JUCE forum](https://forum.juce.com/) is also the best place to file bug
+reports. The JUCE developers are very active there and will read every post and
+respond accordingly.
+
+#### Pull Requests
+
+You must sign the [JUCE Contribution Licence Agreement](https://cla.juce.com/)
+before your code can be considered for inclusion into JUCE. The automated build
+of your Pull Request (PR) will fail if either the author or committer of any
+commits in your PR has not signed the Contribution Licence Agreement.
+
+This repository contains just the public branches of the main JUCE development
+repository. Any work in your PR will not be merged into any other branch in
+this repository, but will instead be reproduced in our private repository. We
+may refactor and rewite any code submitted to us in PRs.
+
+All work should be based on our `develop` branch.
+
+#### Code Of Conduct
+
+All interactions of any kind with the code in this repository or the
+surrounding commentary in Issues or Pull Requests must abide by our [Code of
+Conduct](https://berlincodeofconduct.org/).
+