Firefox requires the form to be attached to a DOM element

ECOM-2492

- Added support for Unicode characters when creating the signature
- Using <textarea> for form data to support newline characters
- Generating form with jQuery instead of relying on existing DOM element
- Removed duplicated credit request code
- Corrected spaing on dashboard
- Enabled functionality by default

Dashboard cleanup
-- Cleaned styles
-- Added wrapper class to use for hiding the actions
-- Cleaned JS toggling of error container
-- Corrected dashboard and receipt text

ECOM-2172

ECOM-2117

ECOM-1578

* Remove m2m relation between credit course and credit providers.
* Separate eligibility and provider APIs into different modules.
* Add API call for retrieving a user's eligibilities.
* Cache credit course list.
* Style the dashboard purchase button.
* Display a link for the credit provider on the dashboard.
* Add analytics events for clicks on the purchase button.
* Expose more credit models to Django admin and add search functionality.