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/Button.tsx

@@ -0,0 +1,21 @@
+import * as React from 'react';
+
+interface Props {
+  children: any;
+  onClick(e: React.FormEvent): void;
+  className?: string;
+  outline?: boolean;
+  disabled?: boolean;
+}
+
+const Button = ({ children, onClick, className = '', outline = false, disabled = false}: Props) => (
+  <button
+    onClick={onClick}
+    className={`${className} btn btn-${outline ? 'outline-' : ''}dark`}
+    disabled={disabled}
+  >
+    {children}
+  </button>
+);
+
+export default Button;