Framework: Use key-mirror in place of react/lib/keyMirror

[aduth]

Related: #786, #787

This pull request seeks to replace existing usage of the react/lib/keyMirror module with the standalone key-mirror package. This is a prerequisite for upgrading to React 0.14. Facebook has moved library modules to a separate package, which they warn against using.

Note: If you are consuming the code here and you are not also a Facebook project, be prepared for a bad time. APIs may appear or disappear and we may not follow semver strictly, though we will do our best to. This library is being published with our use cases in mind and is not necessarily meant to be consumed by the broader public.

https://github.com/facebook/fbjs

Testing instructions:

Ensure that no references remain for react/lib/keyMirror. Verify also that the application continues to load, especially for pages making use of key-mirror functionality.

/cc @blowery 11943-gh-calypso-pre-oss

[blowery]

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