Make expand/collapse buttons consistent in navigation and issuable sidebars

The expand/collapse button in the navigation sidebar is at the bottom of the screen, while on the issuable sidebar it is at the top. We should choose one of the two options and make this consistent.

Navigation Issues and MRs Issue Board
Screenshot_2017-10-11_09.38.13 Screenshot_2017-10-11_09.45.25 Screenshot_2017-10-11_09.45.31

I propose we place it at the bottom for both:

  • It is one of the only configurations that works well for the navigation sidebar
  • It will allow us to dedicate the first row in the issuable sidebar to one purpose only. Currently, the Collapse button lives next to the Add todo button in issues and MRs, and issue titles in issue boards.

cc @tauriedavis @pedroms