Include data keys within rule paths

[jeroenrinzema]

I noticed when working on #724 that the rule paths for data keys are not included within the variable suggestions. It might make sense to somehow store the values for data keys within the database allowing users to rely on the autocompletion in the UI when creating logic gates and/or handlebars templates.

The challenge is in how the rule paths are discovered. Data keys could be set on the entrance step and/or send step. Data returned within the send step could be used within the journey. We could store the keys when testing the webhook within the campaign UI.