Replace hyphen with underscore in Mastodon address

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

app/models/setting.rb

@@ -209,6 +209,6 @@ class Setting < RailsSettings::Base
 
   def self.default_services
     # TODO Make configurable from respective service settings page
-    %w[ discourse gitea mediawiki xmpp ]
+    %w[ discourse gitea mastodon mediawiki xmpp ]
   end
 end

app/models/user.rb

@@ -131,7 +131,7 @@ class User < ApplicationRecord
 
   def mastodon_address
     return nil unless Setting.mastodon_enabled?
-    "#{self.cn}@#{Setting.mastodon_address_domain}"
+    "#{self.cn.gsub("-", "_")}@#{Setting.mastodon_address_domain}"
   end
 
   def valid_attribute?(attribute_name)

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
 
@@ -147,7 +155,7 @@ RSpec.describe User, type: :model do
     after { clear_enqueued_jobs }
 
     it "enables default services" do
-      expect(user).to receive(:enable_service).with(%w[ discourse gitea mediawiki xmpp ])
+      expect(user).to receive(:enable_service).with(%w[ discourse gitea mastodon mediawiki xmpp ])
       user.send :devise_after_confirmation
     end