improve button and add message level 'warning'

Resources/public/sass/custom/_flash_messages.scss

@@ -0,0 +1,14 @@
+.flash_message {
+    margin-top: 2.5em;
+}
+
+// note that other level are defined in modules/_alerts.scss
+
+.alert {
+    // override in modules/_alerts.scss
+   @include alert($red);
+}
+
+.warning {
+   @include alert($orange);
+}