* Add two guards when purchasing credits
This prevents sending an empty card to stripe, and avoids a
foreseeable error when purchasing 0 credits. Error returned from
stripe is "This value must be greater than or equal to 1".
* Move redirect back into action method
Only use validation methods to check input and set appropriate messages.
* Accept organization id if set
The payment service will use the organization id (and charge the
organization) when that's the purchaser. Don't fail when no stripe
token or card selected if an organization admin is purchasing credits.
* Move validation into the payment processing service
This cleans up the number of details the controller needs to worry
about
Now only if the process payment service had any tests...
* Prevent only organization_id from permitting purchase
and move this check to a method, with an intention revealing name
* Add i18n translation for error message
There weren't any service side error messages - this will be displayed
in a view (credits/purchase) but comes from the payment service -
other messages (from stripe) are passed through as-is there.
If this is the wrong _place_ for the translation I can retool.
* Add unit test for payment processing
Only checking errors are raised as expected and the internal error
message is translated.
* Leave translation alone
Revert test translation
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