Varnish VCL cleanup

## Done:

* Cleaned up cookie logic in production.vcl (see #28, and 3fd9d0c)
* Fixed wp cookie check in receive/wordpress.vcl (see 9c2f358)
* Changed varnish to file backend (see #53)
* Removed cache bypassing for local env (see fa96873)
* Removed caching of static files (see 99eb9ad)
* Piping wp-(login|admin) instead of passing (see 89cb137)