Combine drawable files. https://redmine.stoutner.com/issues/794
Update the URL in the copyright header. https://redmine.stoutner.com/issues/796
Bump the minimum API to 23. https://redmine.stoutner.com/issues/793
Fix the app bar and options menu icon colors. https://redmine.stoutner.com/issues/618
Switch to the new Day/Night theme. https://redmine.stoutner.com/issues/522