Fix external user create dropdown empty state

Problem

In self-managed, setting a user as external means they are unable to create. In that case, the UI is shown below with a small empty drodown shown when clicking the (+) button. There is an opportunity to handle this "empty state" better.

Proposal

Set the + button to disabled in this use case.

Edit (@thutterer): I chose to to with "don't show it" instead of "show it disabled", because:

• This keeps the same UX as we had in the old nav. See #420921 (comment 1524765825)
• This is much simpler. If we showed it disabled we would need to explain why it's disabled, probably in a tooltip, and tooltips on a disabled are problematic.