Avoid removing other proposals map locations

It was possible to remove a map location from a different proposal (even
one created by a different author) by modifying the hidden `id`
parameter in the form.

So we're making sure the map location we destroy is the one associated
to the proposal we're updating.

Since we're now using the `@proposal` instance variable in the
`destroy_map_location_association` method, we're calling that method
after loading the resource with cancancan.

spec/controllers/proposals_controller_spec.rb

@@ -8,4 +8,27 @@ describe ProposalsController do
       expect { get :index }.to raise_exception(FeatureFlags::FeatureDisabled)
     end
   end
+
+  describe "PATCH update" do
+    before { InvisibleCaptcha.timestamp_enabled = false }
+    after { InvisibleCaptcha.timestamp_enabled = true }
+
+    it "does not delete other proposal's map location" do
+      proposal = create(:proposal)
+      other_proposal = create(:proposal, :with_map_location)
+
+      sign_in(proposal.author)
+
+      patch :update, params: {
+        proposal: {
+          map_location_attributes: { id: other_proposal.map_location.id },
+          responsible_name: "Skinny Fingers"
+        },
+        id: proposal.id
+      }
+
+      expect(proposal.reload.responsible_name).to eq "Skinny Fingers"
+      expect(other_proposal.reload.map_location).not_to be nil
+    end
+  end
 end