Make flash notification sticky at top to make it always visible

Problem

We should consider having the top flash notification as sticky to make it always visible on the page.

Imagine that in a very long issue with lots of comments, when you perform an action and if it goes wrong on the backend, we show a Flash message at the top of the page but you can only see it when/if you scroll all the way to the top. If we make it sticky, we could see it right away.

Design

• Banner should show up at the top of viewport (let's try 56px from the top of the entire page)
• Banner should be fixed and dismissible
• Spec previews: https://gitlab-org.gitlab.io/gitlab-design/hosted/design-gitlab-specs/error-banners-spec-previews/
• box-shadow: 0 2px 4px 0 rgba(0,0,0,0.25);

Here is a gif to show the issue better.