Show a sample of Identities when creating a Segment

[dabeeeenster]

Overview

When creating a Segment it can be difficult to know if your rule definitions are targeting the intended subset of Identities. It would be helpful to show a sample list of identities for a particular Environment that the Segment is being defined in, and whether they would be part of the segment.

Solution

1. Make the create/edit segment UI take over the primary view of the application, rather than as a modal popup.
2. Split primary view into two parts, vertically separated. The left part of view should be the existing Segment UI (creating rules etc). The right part should show a list of Identities, and indicate whether they match the Segment or not.
3. Every time the Segment definition changes, refresh the right hand list.

API changes

We should add a new endpoint that takes a segment definition, grabs a number of Identities, calculates if they are part of the Segment definition and returns the list to the front end.

Issues

Since we are only able to process Segments in one direction, we have to fall back to sampling of Identities. One potential addition to this feature would be the option to filter the sampling list based on Identity ID. Then users could specify an Identity if they knew an example identity they want to target.