Add a subscribe page and/or a subscription form

[gllmhyt]

Since a few versions, Ghost introduced theme tools for collecting emails from the blog.

When a user enables the "subscribers" feature in labs, several new features are activated:

• The route /subscribe/ will now render subscribe.hbs (provides there is not already a page at that route)
• A new {{subscribe_form}} helper is registered for use within the theme
• The API gets a new /subscribers/ endpoint

It is possible to detect whether or not this feature has been enabled from within a theme, by using the new {{@labs}} helper, please see @labs.

Activated, I can have access to the subscribe page, see: https://postblue.info/subscribe/

But because of the way pages are loaded with Ghostwriter, the page is not loaded when access from the menu bar: