Adds controls to the embeddable carousel.

Adds "Next" and "Previous" buttons to allow for user navigation between embeddables in carousel mode. [#49877927]

https://www.pivotaltracker.com/story/show/49877927
Authoring mode control of embeddable display

[#49877927] This introduces a page attribute for how embeddables are displayed, and a form field in the page edit window to control the setting of it.

https://www.pivotaltracker.com/story/show/49877927