Manage users panel

[ben-kaufman]

Adds a list of existing users and the option to delete each user to the authentication settings page, visible only to the admin.

[stepansnigirev]

Breaks when I try to delete the user. auth key is not in request.form.

These lines need to be moved to if action == "save"::

if current_user.is_admin:
    auth = request.form['auth']

https://github.com/cryptoadvance/specter-desktop/pull/194/files#diff-a386d70e20c16c989ef22377f5ca3398L334-L335

[ben-kaufman]

Ok, I moved that, should be good now.