Adds implementation of GitHub Enterprise suspend / unsuspend user

My company just moved to a site license and I had a need for unsuspending many users. I went ahead and did a first-pass implementation of this but there are no tests.  Before merging this, I am completely willing to go write tests; I just did this in a rush and figured I'd contribute the commit.

Thanks! Let me know if you'd like method naming etc. to be changed.
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