DEV: adds a loading property to d-button (#9072)

Usage:

```
{{d-button icon="times" label="foo.bar" isLoading=true}}
```

Note that  a button loading without an icon will shrink text size to prevent button to jump in size.

A button while loading is disabled.

lib/svg_sprite/svg_sprite.rb

@@ -198,7 +198,8 @@ module SvgSprite
     "user-shield",
     "user-times",
     "users",
-    "wrench"
+    "wrench",
+    "spinner"
   ])
 
   FA_ICON_MAP = { 'far fa-' => 'far-', 'fab fa-' => 'fab-', 'fas fa-' => '', 'fa-' => '' }