Sidebar items no longer reserve space for icons or make the active/hover

indicator appear connected to the content by default. These visuals were

useful for the dashboard, and other pages, but aren't a good starting

point for sidebars.
New options have been added to enable these visuals:
1) Both .rb-c-sidebar and .rb-c-sidebar__section now support

   a -has-icons modifier, which enables the space reservation for icons

   in items.
2) The #rb-ns-ui.sidebars.set-display-style(@style) mixin can be

   called with a value of connected-right to enable the visual of

   connecting to content on the right. We'll be able to extend this down

   the road to offer other visual styles, if needed. The page sidebar

   currently turns this on. Future changes may change that default as

   well, depending on needs.
Templates can add modifiers (or entire new CSS classes) to the main

.rb-c-sidebar through a sidebar-css-classes template block. The

datagrid template takes advantage of this to enable icon reservation.
There's also a fix for setting the width of a sidebar from within a

rule for a page CSS class. Previously, sidebar widths could only be

set at the top level of a stylesheet.

Verified that the main page sidebar for the dashboard looks as it did

before this change, but that other sidebars (in upcoming changes) didn't

have the space reservation or connected style by default.