Changed document structure of all four collections

Each item now has a SKU so each item can be branded (takes the place of
“product_id”) and a list of categories for searching purposes. Items
are no longer specific to certain stores.

Each record points to the id of the user that added it, the id of the
item it’s for, and the id of the grocery list the record was a part of.
Record also has a quantity and whether or not a loyalty card was used
when purchasing the item at the record’s price. The saleType field is
now more general. Some saleType examples include ‘in_store’,
‘store_coupon’, and ‘manufacturer_coupon’.

Stores are no longer specific to certain users.

Users now have an array of pendingRecords which will be used in the
creation of a grocery list. Users also have an array of storeIds to
keep track at which stores in the collection of stores the user has
shopped.