Enable switching servers while already logged int

[JAicewizard]

What

When logged in, allow switching servers.

Why

This would make it very easy to switch servers. This option can be hidden if not logged into any other backend, to avoid confusing users

Prerequisites

• allow multiple logins at the same time (and listing logged in accounts)

Context

This would allow opening both thalia.nu and staging.thalia.nu links in the app without getting errors or switching accounts manually. It could automatically detect which backend to sue