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)
Edited by Serhii Yarynovskyi