The new design has the events list on the left and the calendar on the
right, so swap them around and remove the vertical separator between
them in favor of some additional whitespace as in the mockups.
https://bugzilla.gnome.org/show_bug.cgi?id=744817
The sass sources now live in a project in GNOME, so they can
be used in multiple projects like gnome-shell-extensions.
Because of that, add gnome-shell-sass as a submodule of gnome-shell
and remove the sass sources from the theme directory.