Allow users to subscribe to email alerts for a finder

For all finders we want to allow users to sign up for email alerts when new documents are published in the formats that finder deals with. This includes two separate pieces of work:

*  [Ticket 1 -- Sign up page displayed for each finder](https://trello.com/c/2KUtZWCp/322-email-alerts-sign-up-page-for-a-finder-document-format-3)
* [Ticket 2 -- Allowing users to sign up](https://trello.com/c/lKCpZFVv/323-email-alerts-users-can-sign-up-from-a-email-sign-up-page-3)

**Screenshot**
![screenshot](https://dl.dropboxusercontent.com/u/39836361/Screenshot%202014-09-18%2015.43.35.png)

The copy for the second sentence on this page is provided by the `metadata/:finder-slug.json` files created in https://github.com/alphagov/finder-api/pull/55

To complete before merge:

- [ ] Merge and deploy https://github.com/alphagov/finder-api/pull/55 ensuring rake task is run to create the new artefacts
- [x] Complete investigation into `gov_delivery` API erroring when data differs between local and remote -- might not be a problem outside the development environment. *(wasnt a problem anywhere except development)*
I'm not sure this is a good idea. It's more RESTful but is a pretty crappy URL for users. What about using GET and POST to the same route?