Moving our CLA in-house

[john-science]

What is the change?

The Contributor License Agreement (CLA) is the agreement everyone must sign before they can make a PR into ARMI.

Why is the change being made?

But the text for our CLA was previously saved to a private gist under the user youngmit's account. As this is an important ARMI document, I believe ARMI should have control over it. So I have moved the document into the ARMI repo.

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify any new/changed code.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes have been updated if necessary.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in pyproject.toml.

[opotowsky]

Not a lawyer, but looks good to me!