Allow advanced json styling for Stripe CC field

I attach a patch with a possible solution. The style setting field has been increased in size to a textarea to allow for more realistic styling options. As it does not make sense to write a full-on CSS parser in this module just to pass the styling to uc_stripe.js, this patch checks if the value provided in this field has already been encoded as valid JSON or not. If it has, it passes the JSON as-is to uc_stripe.js. If the string is not valid JSON, it falls back to the way this module has worked before, that is, it will use the helper function _uc_stripe_get_element_styles_array to convert a simple, comma-delimited list of style:value pairs into an array, before encoding it as JSON. I have updated the admin UI descriptions, too.

In other words, with this patch applied this module works the same way for existing installations that use the simpler styling strings, while adding the option of more complex formatting if desired.

On another note, it might be a good idea to change the default style from using 1em font size, as that generates a Stripe warning, since Stripe now recommend using absolute font sizes in px, rather than relative. I would be happy to add that change to the patch if @AndraeRay agrees.

Good idea, @AndraeRay. Do you mean a new text area and to remove the existing one, instead of changing the existing one? If we do that, it will break sites that have used Features to export the existing one, as the name of the variable will change. That is the reason why I have repurposed the existing one.

Adding a checkbox or a toggle of some other kind to let the user decide between "simple" and JSON-encoded makes sense. It would allow us to do a simple validation of the JSON if that is the intention of the user, catching simple errors that are hard to spot like incorrectly pasted quotation marks—don't ask why. :) The toggle would need to default to the simple style so as not to break existing sites esp. if they use Features. What do you think?

I understand. I will need to dig up how to create a toggling admin form field. Give me some time. :)

I attach a new patch which creates a new field and a variable for the JSON-encoded styling. I added form validation that checks if it is valid JSON. Also, I made a change to uc_stripe.js so that the new styling becomes the value of the "style" element, rather than its "base" child. This allows for richer styling, for example including complete, empty, and invalid variants, which was not possible before. At the same time, there is no change to how simple styling works because after the simple string gets JSON-encoded it gets surrounded with the "base" element, unless it is empty.

I have also updated the help text's link to the Stripe site because the old one was no longer pointing to the styling section.

Finally, I have changed the default styling to use fontSize: 16px instead of 1em as per the new Stripe recommendations—using relative metrics, like 1em, now throws a warning in the browser debug console.

On another note, @AndraeRay, my Drupal coding standards editor plugin tells me that there are minor indentation and whitespace issues in the current version of uc_stripe.module and uc_stripe.js. If you would like me to, I can regenerate this patch with the indentation styling consistent with Drupal standards. Your decision.

Thank you for your help, @AndraeRay.

Many thanks, @AndraeRay, for working on this issue and for the commit. :)