Support adding and removing segment to a component variant

REST API:

• Support getting a list of existing segments
• Support linking a segment to a component variant
• Support unlinking a segment to a component variant

UI:

• Show button that will allow a user to link a segment to a variant when creating one
• Show a list of segments to be linked in a dialog for the user to choose from

Designs:
https://bloomreach.invisionapp.com/console/UX-issues-ckemg823415qp01wla1eu1m86/ckemgd91q08df01zte7ugjn63/play