Fix module/class resolution bugs in SmartdownAdaptor

This was only happening in production mode, not dev, due to different class loading/caching behaviour.

Because the App presenters (for smart_answers) aren't namespaced by a module they will
get picked over SmartdownAdaptor versions, due to ruby scoping/resolution rules (related, http://techblog.thescore.com/how-you-nest-modules-matters-in-ruby/)

This commit makes sure we explicitly use SmartdownAdaptor:: classes. This will need to be done for any future use of
SmartdownAdaptor classes that share a name with top level classes, which is mostly smart_answer Presenters, as they're
not namespaced into the SmartAnswers module, they probably should be.

For now this just fixes the instances breaking smartdown flows in production, so we can continue testing.

I'm going to add work to the Smartdown backlog to fix the broader name spacing issue separately.
Merge pull request #1104 from alphagov/fix-class-name-resolution-bug

Fix module/class resolution bugs in SmartdownAdaptor