Feature/re enter credentials to use admin with session store

What does this MR do?

This MR is like a proposal to add a admin_mode and a user_mode for admins. See: https://gitlab.com/gitlab-org/gitlab-ce/issues/23259

A user_mode attribute is added to the session and only allows site admin features to be used, when the user is in admin mode.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

This MR is thought of as a mini prototype to suggest a way how to introduce a amin_mode.

The development of this MR is sponsored by @ siemens (/cc @bufferoverflow @koffeinfrei).

Screenshots (if relevant)

Indicator when in admin_mode and button to leave admin_mode:

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?