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/Spinner.tsx

@@ -0,0 +1,10 @@
+import * as React from 'react';
+import I18n from 'i18n-js';
+
+const Spinner = ({ color = 'dark' }) => (
+  <div className={`spinner-grow d-block mx-auto text-${color}`} role="status">
+    <span className="sr-only">{I18n.t('common.loading')}</span>
+  </div>
+);
+
+export default Spinner;