:end_date, :interval_length and :interval_units are stored on the
subscription line items before the order is completed.

Once the order is completed we want a canonical source for these values.
These values will now be stored on the subscription itself, once it is
created

Add a method to group subscription line items by their subscription
configuration attributes:

- interval_length
- interval_units
- max_installments

This method should be used to group an order's subscription_line_items
into groups which can be fulfilled by a single subscription.

When creating subscriptions, persist the configuration values present on
the subscription_line_items on the subscription itself

Intervals have previously been stored as an enum on the
subscription_line_item model. They will now also be stored on the
subscription model.

Both models need to understand and calculate these durations in exactly
the same way.

This commit extracts the enum, validations and supporting methods into a
module which can be shared between these two models.

A separate subscription should be created for each group of similar
subscription line items.

ex. subscription line items with different fulfillment intervals cannot
be fulfilled by the same subscription. But if the line items have all
the same subscription configuration option, they can be fulfilled by the
same subscription

When referenced as a variable in specs

Add a spec to test that the checkout is successful when the subscription
includes multiple line items