closes#6984
-the backlink had a static href to {{@blog.url}} which is now changed to {{#if subscribed_url}}{{subscribed_url}}{{else}}{{@blog.url}}{{/if}} to reflect the referring url.
-This PR makes the back link on the subscribe page to link back to the referring page instead of linking back to /
Form:
- add confirm, location & referrer hidden fields
- add script to populate location & referrer
- add helper for creating the email field
- pass through input class and placeholder for email from top level form helper
- rename subscribe_form template & helper as it sounds more natural
- handle success and error cases differently
- improve error message display
- ensure useful data is passed back so that we can show nice messages
- check for honeypot value being filled out
- refactor error handler to set an error and always still render
