FIX: Assignments limit shouldn't prevent from reassigning a post (#556)

At the moment, it's possible to have maximum 5 assignments per topic 
(that includes topic and post assignments). When trying to assign more, 
a message "Limit of 5 assignments per topic has been reached" appears.

One possible edge case here is _reassigning_ a topic or a post. Reassignment 
doesn't lead to exceeding the limit, and therefore it should be possible. But 
at the moment we handle correctly only _topic_  reassignments, while when 
reassigning a _post_, we show the error message "the limit has been reached".

This commit makes post reassignments work correctly too.

lib/assigner.rb

@@ -547,8 +547,7 @@ class ::Assigner
   end
 
   def reassign?
-    return false if !@target.is_a?(Topic)
-    Assignment.exists?(topic_id: @target.id, target: @target, active: true)
+    Assignment.exists?(target: @target, active: true)
   end
 
   def no_assignee_change?(assignee)

spec/lib/assigner_spec.rb

@@ -264,6 +264,21 @@ RSpec.describe Assigner do
         expect(second_assign[:success]).to eq(true)
       end
 
+      it "reassigns a post even when at the assignments limit" do
+        posts =
+          (described_class::ASSIGNMENTS_PER_TOPIC_LIMIT).times.map do
+            Fabricate(:post, topic: topic)
+          end
+
+        posts.each do |post|
+          user = Fabricate(:moderator)
+          described_class.new(post, admin).assign(user)
+        end
+
+        status = described_class.new(posts.first, admin).assign(Fabricate(:moderator))
+        expect(status[:success]).to eq(true)
+      end
+
       context "when 'allow_self_reassign' is false" do
         subject(:assign) do
           assigner.assign(moderator, note: other_note, allow_self_reassign: self_reassign)