Framework: Initialize localStorage polyfill at start of boot

[aduth]

Fixes #6555
Suspected regression in #6236 (lib/abtest pulls in store)

This pull request seeks to resolve an issue where Safari Private Mode does not load correctly because of an attempt to access localStorage (not available). Our localStorage polyfill is targeted at resolving this case, but must be loaded before any dependencies making use of store or localStorage. This pull request moves the initialization to the top of the boot file to ensure that it is always executed first.

Testing instructions:

I was never able to reproduce the original issue in development mode locally, perhaps due to some configuration flag that's only active in production (makes sense if it is ABTest related). You may want to search internal documentation for running production mode locally to verify the fix (which I did to verify resolution), else ensure that no regressions occur in using the application. Specifically, ensure that you can load Calypso in Safari Private Mode.

/cc @rachelmcr, @bisko, @blowery, @mjuhasz

Test live: https://calypso.live/?branch=fix/6555-localstorage-private

[rachelmcr]

I confirmed that Calypso loads for me in Safari Private Mode on the calypso.live branch.

[aduth]

@rachelmcr : Do you see it being broken at https://calypso.live/?branch=master ?

[bisko]

Confirmed broken on the calypso.live master branch.
Confirmed working on the calypso.live fix/6555-localstorage-private branch.

Unable to reproduce on local Calypso, because it seems that localStorage has a bit of weird behaviour on local sites.

[rachelmcr]

Hm, I checked the master branch on calypso.live and it's working for me there, so at least on my end I can't confirm the fix based on calypso.live testing.

[aduth]

Ref: 2834-gh-calypso-pre-oss (cc @rralian )