Bug 1756764: Alert user if shared with rules 2nd option empty

This is for the 'Search for...' Friends / Groups / Users

Currently we don't allow the submission / cancel of the form if any of
these 3 options are selected but the second select2 box is empty

But we also don't tell the user why.

I've added an alert to mention why the form will not submit / cancel

behatnotneeded

Change-Id: I71827647fd7a2f8177c336bdb91f6ce8b9b23df8
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>