Internal user multiple orgs -- prevent logging in as "no org"

[CarlosNZ]

It's possible to have multiple internal orgs, and have users belong to more than one of them. But when they log in, they are presented with the usual "select org" dialog, which also includes "Log in without organisation".

On the main app, we explicitly prevent internal users selecting "No org" from the menu bar org selector, so we should probably do the same here for consistency.

I would go a step further and define this as a system-wide preference (i.e. in preferences.json), with the default behaviour that internal users cannot login without an org, but can be configured to on a system where that might be allowed.