Replace hyphen with underscore in Mastodon address

Unfortunately, Mastodon only allows underscores for usernames, and
reversely, akkounts only allows hyphens and no underscores.

spec/models/user_spec.rb

@@ -41,6 +41,14 @@ RSpec.describe User, type: :model do
           expect(user.mastodon_address).to eq("jimmy@kosmos.social")
         end
       end
+
+      describe "username contains hyphen/dash" do
+        let(:jammy) { build :user, cn: "jammy-jellyfish", ou: "kosmos.org" }
+
+        it "returns the user address" do
+          expect(jammy.mastodon_address).to eq("jammy_jellyfish@kosmos.org")
+        end
+      end
     end
   end