dmails: add dmail sending rate limits.

Don't allow regular users to send dmails to more than 10 different
users in one hour. This is an anti-spam measure.

test/unit/dmail_test.rb

@@ -69,7 +69,7 @@ class DmailTest < ActiveSupport::TestCase
     should "create a copy for each user" do
       @new_user = FactoryBot.create(:user)
       assert_difference("Dmail.count", 2) do
-        Dmail.create_split(from: CurrentUser.user, creator_ip_addr: "127.0.0.1", to_id: @new_user.id, title: "foo", body: "foo")
+        Dmail.create_split(from: CurrentUser.user, creator_ip_addr: "127.0.0.1", to: @new_user, title: "foo", body: "foo")
       end
     end
 
@@ -104,6 +104,21 @@ class DmailTest < ActiveSupport::TestCase
       end
     end
 
+    context "sending a dmail" do
+      should "fail if the user has sent too many dmails recently" do
+        10.times do
+          Dmail.create_split(from: @user, to: create(:user), title: "blah", body: "blah", creator_ip_addr: "127.0.0.1")
+        end
+
+        assert_no_difference("Dmail.count") do
+          @dmail = Dmail.create_split(from: @user, to: create(:user), title: "blah", body: "blah", creator_ip_addr: "127.0.0.1")
+
+          assert_equal(false, @dmail.valid?)
+          assert_equal(["You can't send dmails to more than 10 users per hour"], @dmail.errors[:base])
+        end
+      end
+    end
+
     context "destroying a dmail" do
       setup do
         @recipient = create(:user)