Add migration service

* Checks migration flag has not been set and app version === v0.10.4
* If so, reads in entire Chrome storage,
* … adds known collections to Pouch,
* … and sets a flag (in Chrome storage)

(Does not delete anything in Chrome storage).

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Tested via:

* Clearing Pouch and local storage
* Checking out v0.10.3
* Setting a (bogus) stock count
* Checking out develop
* Confirming stock count match