Bug 870502: Ajaxify newsletter forms
Bug 870502: Add newsletter thank you CSS animation
Bug 870502: Fix contribute page newsletter form.

* Add basket failure tests for subscribe view.
Bug 870502: Fix contribute form JS and tidy up newsletter CSS

* Tidy up some more newsletter CSS
* Address minor JS review comments
Merge pull request #2193 from pmclanahan/ajaxify-newsletters-870502

Bug 870502: Ajaxify newsletter forms