Add `DSIABLE_SSL` env option

[kevinmcconnell]

assume_ssl and force_ssl are often used together:

• When running behind a terminating proxy (including Thruster) you'll want both.
• When running without SSL (like on localhost) you'll want neither.

To simplify setup for those cases, we add the DISABLE_SSL option. When set to true all SSL-related behaviour is turned off. When left as the default, it's on. This is handy for self-hosting with the Docker image, as it's one less ENV var to have to set.

We can still use ASSUME_SSL and FORCE_SSL to set those separately if required.

cc @lewispb