Show a "there are new, inactive users" hint

[philippotto]

At the moment, it's quite hidden that inactive users need to be activated. Show a more prominent (but not too much in-your-face) hint.

[fm3]

related: https://discuss.webknossos.org/t/no-notification-for-new-user/884

[normanrz]

I think it would be great to list those users directly for the admin to activate. These users should only be newly created users (say last 2 weeks).