Remove fixed position of invite acceptance alert on sign up page

What does this MR do and why?

When an invited user clicks the "join team" CTA in their GitLab invitation, they are taken to the /signup page and prompted to either create an account or sign in to GitLab via an alert. The positioning of this invite acceptance alert is currently fixed and is causing usability issues in some formats including mobile

Screenshots or screen recordings

https://drive.google.com/file/d/1K9WxDwzPvUn7xnfbvpM1QU0sasvBruyq/view?usp=sharing

Related to #348740 (closed)