FIX: Require a min amount of reviewables before calculating thresholds

On forums with very few flags you don't want to calculate averages
because they won't be very useful. Stick with the defaults until we hit
15 reviewables at least.

app/jobs/scheduled/reviewable_priorities.rb

@@ -3,7 +3,12 @@
 class Jobs::ReviewablePriorities < Jobs::Scheduled
   every 1.day
 
+  def self.min_reviewables
+    15
+  end
+
   def execute(args)
+    return unless Reviewable.where('score > 0').count >= self.class.min_reviewables
 
     # We calculate the percentiles here for medium and high. Low is always 0 (all)
     res = DB.query_single(<<~SQL)