Purchases: Move purchases logic in the API

[stephanethomas]

As discussed in #2602 (comment), we shouldn't have that much logic in our purchases functions. To be more specific the logic that determines whether a purchase is cancelable or is removable should live in the API. As such, we should take advantage of the is_cancelable property returned by the endpoint which we don't use at all in Calypso.

We should also take this opportunity to remove any unnecessary property from the purchases assembler such as subscribedDate or tagLine - there is no need to keep them around.

[stale]

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