GitLab for Jira app should enforce authentication and display who is logged in

Problem to solve

The majority of the errors in the installation process occur during the configuration step, when a user adds a namespace. This is partially due to the fact that the user doesn't know which namespaces they have access to. It is not clear who they are logged in as in GitLab or whether or not they are even logged into GitLab.

Propsal

• Add a way to sign into GitLab
• Expose who is currently signed in
• Add a way to sign out (in case the user has multiple GitLab accounts)

Mockups in Designs section.

• 👉 Figma prototype - First run
• 👉 Figma prototype - Managing an existing setup

See (#232727 (closed)) for an example of entire flow installation flow.