Added CSS rules to make sure that the sidebar is always visible for browser widths above the sm- cutoff point.

This is intended to fix the following bug:

- the user opens the app in a desktop browser
- the user reduces browser width until the smartphone style is applied (remember, Bootstrap decides whether to switch to smartphone layout based on browser width, not user agent)
- the user clicks on a sidebar feed to read it. At this point the Velocity.js library adds a style="transform: translateX(-100%)" attribute to the sidebar, which makes it disappear to the left of the viewport.
- the user resizes the browser back to normal desktop width.

At this point, with the old behavior, the sidebar (which had been hidden while in smartphone mode) stayed hidden. This broke the expected behavior: the sidebar should always stay visible at browser widths abouve the sm- cutoff.

This commit fixes this bug. When resizing the browser back to normal desktop width, the sidebar reappears immediately.