Conversion collections models now have a step property

The step property exposes the step name indepentley from the actual
properties returned by backdrop api.

@abersager, @maxfliri
Step initialisation can be overridden

@abersager, @maxfliri
Use step property in conversion-success-rate view

@abersager, @maxfliri
LPA conversion collection gets step from eventLabel property

@abersager, @maxfliri