Configure which lockdown browsers are available site-wide

[christianp]

At the moment, the "require a lockdown browser" setting for a resource always shows the Numbas app and Safe Exam Browser.

It should be possible to configure which apps, if any, appear in this list, to match institutional support.

There should be a list AVAILABLE_LOCKDOWN_APPS in settings.py, whose default value is ["numbas", "seb"], determining which options are offered. If the list is empty, the setting should not be shown.