Testing for close buttons
Refactoring and testing neighbor-finding methods
Fixing spec tests to rm magic numbers
Add "Begin" to "Next Activity" [#49305717]

https://www.pivotaltracker.com/story/show/49305717
Refactoring video_interactives_controller

Reducing complexity and repetition.
Refactoring Embeddables models to be more DRY
Simplify finding an embeddable's activity
Refactoring some controllers to reduce complexity
Make sure interactive doesn't scroll over buttons

[#49305717]
Refactor to simplify in controller