Changed the purchase helper to use the default storage location
of the organization first, before using any other storage locations
that are available.

Also added a (failing) spec to check that we can see the option
on the page. Should be fixed in the next commit

Fixed the failing test by using the extended format
of the test expectation and used `subject` to make
an assertion regarding the response body.

Added a test that when we begin to make a new donation,
we have a default storage location set.

Switched to use the helper method from PurchasesHelper
to set the default storage location for a donation

…onation

Addressed review feedback namely:
* Renaming new_purchase_default_location to set_default_location
and moving it to ApplicationHelper. This allows us to use across
both the donation and purchase forms.
* Updated the tests and forms accordingly
* Added specs to ApplicationHelper to test that we are able
to successfully set the default storage location for purchases
and donations.

Renamed the method as per review suggestion
and keep only one test since we only need to test
that the default storage location is successfully set

…onation

Updated spec to match the new behaviour of
the donation form.