Added property prompts to xblock.

TNL-708
Parse and serialize multiple prompts

TNL-708
Fixed xml tests.

TNL-708:wq
Added tests for prompts field.

TNL-708
Added tests for xml parsing & serialization.

TNL-708
Created studio editor templates for handling multiple prompts.

TNL-708
Sass for Studio multiple prompts editor.

TNL-708
Studio editor can edit multiple prompts.

TNL-708
Updates to studio_mixin

TNL-708
Fixed studio tests

TNL-708