LPA conversion funnel

@YolinaS
@maxfliri
Feature toggle for LPA conversion funnel

@YolinaS
@maxfliri
Generic multiple collection for conversions

@YolinaS
@maxfliri
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
Copy update for LPA conversion funnel module
Fix failing test

@maxfliri
Remove from fixture properties that should be ignored

@maxfliri
Merge branch 'master' into lpa-conversion-funnel

Conflicts:
	app/controllers/backdrop_stub_controller.rb
Merge pull request #127 from alphagov/lpa-conversion-funnel

Lpa conversion funnel