Show last access information for mail users - frontendonly

Helmorequested to merge
helmo/ispconfig3:5374-mail-last-accessed-frontend into develop

This is an alternative to !1788 (closed)

It's just the frontend code, updating data in the table needs either the dovecot code from !1788 (closed) or a separate cronjob as described in #5374 (closed)

Closes #5374 (closed)

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