upload_widget: Use img tag for tail-spin.svg instead of object tag.

We use img tag for showing the loading spinner in image_upload_widget
instead of object tag because using object tag gives the following
error "Refused to display 'https://chat.zulip.org/' in a frame because
it set 'X-Frame-Options' to 'deny'." in console.

As mentioned by Anders in czo, this is probably because we’re loading
the SVG using an <object> tag as if it’s totally-not-an-IE3-ActiveX-
control.

Discussed in #issues > loading tail-spin.svg denied by X-Frame-Options.

static/templates/settings/image_upload_widget.hbs

@@ -11,7 +11,7 @@
             {{ upload_text }}
         </span>
         <span class="upload-spinner-background">
-            <object class="image_upload_spinner" type="image/svg+xml" data="/static/images/tail-spin.svg"></object>
+            <img class="image_upload_spinner" src="/static/images/tail-spin.svg" alt="" />
         </span>
     </div>
     <img class="image-block" src="{{ image }}"/>