Port Compare page to the useEffect hook

[kumar303]

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The Compare page grew some gnarly load/update logic in https://github.com/mozilla/addons-code-manager/pull/342

This component can be ported to useEffect which will simplify the loading logic and let us delete a bunch of the tests.

┆Issue is synchronized with this Jira Task

[willdurand]

This could be done together with #111.

[willdurand]

I guess this is blocked by upstream for now because Enzyme (shallow) does not support Hooks: https://github.com/airbnb/enzyme/issues/2011.

[kumar303]

If we only want useEffect, we might only need to wait on https://github.com/facebook/react/issues/15275