Allow multiple tabs by not creating a new csrf token if one is already present

Description

This PR slightly changes the implementation of the get csrf token endpoint to return the already present csrf token if there's one.

[x] I added tests to new or existing code
[ ] I removed hardcoded strings and used our i18n solution instead (see here)
[x] I made sure no sensitive info gets logged at any time in the codebase (see here) (e.g. no user info or details, no stacktraces, etc.)
[ ] I checked that infrastructure/update_infrastructure.sh runs without any error
[x] I checked that npm run build builds without any error
[x] I checked that clusters are listed correctly
[x] I checked that a new cluster can be created (config is produced and dry run passes)
[x] I checked that login and logout work as expected

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