Add plugin factory - plugins specific for each flow

This changes how plugins are defined. Now, instead of defining them as classes in a huge Plugins object, they are defined in /lib/smartdown_plugins/<flow_name>.rb

See https://github.com/alphagov/smart-answers/blob/2433e10524d0c6324772d8ddfbd0dc19f1d31e8f/lib/smartdown_plugins/animal-example-multiple.rb for an example.

I think that would definitely break things when we have two salary questions on one page: no instance of that happening in the current smartdown flows so won't break things, but we should put in a story to fix that if we're not addressing as part of this PR. 
Unrelated note: have we got an extra story in the backlog for addings years to the salary dropdown?