Create a general component for the reactive sidebar

[elstoc]

Should incorporate a close button at the top and render children either inside a div or inside a custom drawer (or equivalent) component depending on an input prop.

Start by just refactoring the current functionality and then flesh it out later.

[elstoc]

• [x] Get existing sidebar/nav working inside the ContentWithSidebar component
• [x] Have MarkdownToolbox and VideoToolbox emit a single div containing the icons instead of a toolbar component. For the VideoToolbox just separate the two groups of icons with a margin or something
• [x] Remove the "center" option from the Toolbox component
• [x] Amend ContentWithSidebar to use the Toolbox component to compose its own toolbar. The passed-in toolbox should appear on the right and the (optional) nav button on the left
• [x] Remove outer margin styling from Video & Markdown sidebars
• [ ] Create independent margin styling for single-pane and two-pane sidebars
• [ ] Add some sort of close button to the mobile sidebar
• [ ] Extend the drawer to 90-95%