Adjusting navigation options on activity show

Removing the "Next activity, last activity" buttons. Adds a "Back to sequence" button which only appears if the activity is part of a sequence; if that's present, floats the "begin activity" button right, otherwise leaves it in the center. [#49305717] [#53812235]

https://www.pivotaltracker.com/story/show/53812235
https://www.pivotaltracker.com/story/show/49305717
Hiding "next page" and "last page" when disabled
Cleaning up process of figuring out "next" activity
Positioning buttons at bottom of last page

This fixes [#57596514]. Also, in cases where an activity is part of a theme, there is an additional "Back to #{theme.name}" button, which is (so far) awkwardly positioned; this is for [#53812235] and [#49305717].

https://www.pivotaltracker.com/story/show/57596514
https://www.pivotaltracker.com/story/show/53812235
https://www.pivotaltracker.com/story/show/49305717