Finish preview setup and spec testing for authors.

[#60623214] This sets up a /preview link for both activities and pages which will clear all answers from the related Run, then display the activity/page as usual.

https://www.pivotaltracker.com/story/show/60623214
Changes "run" links in authoring mode to use Preview.

Also changes the relevant link text from "Run" to "Preview" where applicable, and ensures these actions use the proper layout. [#60623214]

https://www.pivotaltracker.com/story/show/60623214