Feature 425. Multiple account sign in and account switching

[MytsV]

Partial implementation for #425

• Improve the structure of the account dropdown
• Add a possibility to sign in multiple times
• Handle switching the active user in the session
• Seemingly fix the logout redirection issue (Logout: Redirects #422) from the viewpoint of a user

Needs integration with the multiple account modal for completion

[MytsV]

TODOs:

• investigate the issue with logout/switch not working until the page is refreshed
• show an error if the user tries to log into an already authenticated account
• don't display the current account in the multiple account modal

[MytsV]

• logout from all button

[MytsV]

The error in the build is caused by an intentional choice of using the <a> element instead of <Link>, as it prevents a bug caused by Next.js route caching.
The user couldn't log out in some unpredictable scenarios as the router just performed the redirect without the logic underneath it

[MytsV]

#425 finished fully

[MytsV]

This fixes #422 as well