Allow to programatically re-enable atmo on mobile devices

[tilsammans]

This way, sites have the option to force this value after the Pageflow
javascript-code is done initialising.

See #554 for previous discussion.

[tilsammans]

@tf what do you think of this change?

It would allow us to enable the value in the application.js, thereby ignoring the default.

[coveralls]

Coverage remained the same at 99.053% when pulling e2f36f4 on scrollytelling:atmo-disabled-config into bfe5e70 on codevise:master.

[tf]

I guess that's fine. It should then be safe to call pageflow.atmo.enable() when the pageflow.ready promise resolves.

[tf]

The help texts will still claim that atmo is disabled on mobile, though. But the medium term goal has to be making atmo work on mobile anyway since technically it should be possible by reusing audio elements.

[tilsammans]

Just to be sure, that would be this code, right?

pageflow.ready.then(function(result) {
  pageflow.atmo.enable();
});

[tf]

I think so, yes.