Improve the pattern used for download artifact button on MR widgets

🧩 Problem statement

The download button with a dropdown appears on the MR widget even when there's no artifact present. The dropdown list presents an empty state. This is deteriorating the experience because it sets a false expectation for users to find an artifact.

✨ Proposed solution

Add a visual state to communicate to the users that artifacts are being loaded and present an active download button only if it exists. (We put the action under a kebab menu on the pipeline index page, but that wouldn't work here as there are no other actions to be grouped together)