Extend facebook-autosignin feature switch indefinitely

@jamesgorrie and @chrisfinch have shown me that the switch still has effect
here:

https://github.com/guardian/frontend/blob/cd94607f9/common/app/assets/javascripts/bootstraps/common.js#L413

...so I no longer want to remove the switch!:

https://github.com/guardian/frontend/pull/3687

(because Facebook is obviously a 3rd party system and we want to be able
to turn off dependencies on those quickly)
Merge pull request #3706 from guardian/extend-facebook-autosignin-switch-indefinitely

Extend facebook-autosignin feature switch indefinitely