mitigate breach attacks using django-debreach

BREACH is a category of vulnerabilities and not a specific instance affecting a specific piece of software. To be vulnerable, a web application must:

  * Be served from a server that uses HTTP-level compression
  * Reflect user-input in HTTP response bodies
  * Reflect a secret (such as a CSRF token) in HTTP response bodies

Since horizon falls under this category, we can include django-debreach module within horizon as a requirement which provides mitigation against the breach attacks.

https://github.com/lpomfrey/django-debreach

CSRF token masking is a built-in feature within Django 1.10+, therefore only content-length modification feature provided by django-debreach can be enabled.