Add (and remove) event hosts from the user management page

[tompollard]

Currently, users are added as "hosts" manually (via the shell). We should consider adding an "invite as host" button to the user management page in the console.

The general idea is that we would go to https://physionet.org/console/user/manage/SLUG/ and click "invite" as host. This would send an email to the user explaining the event functionality and offering a link to sign up as a host.