Fix query for nagios alerts on unsent social media broadcasts

Fix query for nagios alerts on unsent social media broadcasts

As of 45b8e2e9, we update the sent column to be True while leaving
unsent providers in the linked table (so we can keep track of which
succeeded and which failed). The nagios check never got the memo about
that one, so it didn't check the sent column, only the linked table,
when in fact it's now enough to *just* check the sent column. So do
that.