WIP on making a 'preview' action which calls 'show'.

[#60623214] This should let an author preview as though they were a student visiting the activity for the first time (with no existing answers).

https://www.pivotaltracker.com/story/show/60623214
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
Update controller/view specs for Preview. [#60623214]

This is mostly view stuff, so I'm taking this opportunity to move it out of the controller spec and put it in its own view spec.

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