Adds journey step type to remove a user from a journey

[pushchris]

Currently once a user starts a journey there is no way to remove them save for the API or having lots of gates that just branch a user off. This PR explores one possible solution which is to allow for a step type that can completely remove a user from a journey. The envisioned way this would be used is to create a secondary path (a new entrance) which would be considered the "exit criteria" path. If a user meets whatever requirements are in place by that path, you can then chose to remove them from the journey entirely. This would allow for almost infinite flexibility in how "exit criteria" is determined.

[williamhrs]

Hey @pushchris we were planning on doing that on our side here, do you have the plan to finish this anytime soon? otherwise we will work on something else