Setting to 100% width

This adds a new configuration to the `extra.css` such that the body of the page fills the full width of the web browser, rather than having a fixed width. The wide margins visible with larger screens disappear.

This was discussed in issue #46 and in the software coordination meeting.

docs/stylesheets/extra.css

@@ -18,3 +18,8 @@
     padding-top: .1rem;
     padding-bottom: .1rem;
 }
+
+/* Fill the width of the browser window, no margins */
+.md-grid {
+    max-width: 100%;
+}