On last page of activity, "Next" is "Back to TOC"

[#53812235] [#49305717] This should make it easier for students to go from the last page to the two places they'll hop to the next activity: the activity TOC page and activity summary page.

https://www.pivotaltracker.com/story/show/49305717
https://www.pivotaltracker.com/story/show/53812235
Next activity and Last activity in activity TOC

[#53812235] [#49305717] Links for the next activity in the sequence and the last one, showing when the activity is in sequence context. This is on the TOC; a subsequent commit will use the same partial in the summary page.

https://www.pivotaltracker.com/story/show/49305717
https://www.pivotaltracker.com/story/show/53812235
Holding feature test out of Guard
Puts summary page in runtime layout

Also adds "next activity" and "last activity" buttons for [#53812235] [#49305717]. Renders the Activity Summary with the runtime layout, and brings over the CSS necessary to do it.

https://www.pivotaltracker.com/story/show/49305717
https://www.pivotaltracker.com/story/show/53812235