Refactor CSS (#116)

Refactor CSS files and structure. Also refactors some html and React components for a smarter use of CSS classes.

app/javascript/components/common/Sidebar.tsx

@@ -0,0 +1,13 @@
+import * as React from 'react';
+
+interface Props {
+  children: React.ReactNode;
+}
+
+const Sidebar = ({ children }: Props) => (
+  <div className="sidebar">
+    {children}
+  </div>
+);
+
+export default Sidebar;