Turn default services into a configurable setting

With the default value being all enabled services

spec/models/setting_spec.rb

@@ -0,0 +1,23 @@
+require 'rails_helper'
+
+RSpec.describe Setting, type: :model do
+
+  describe ".available_services" do
+    before do
+      Setting.discourse_enabled = true
+      Setting.ejabberd_enabled = true
+      Setting.email_enabled = false
+      Setting.gitea_enabled = false
+      Setting.mastodon_enabled = true
+      Setting.mediawiki_enabled = false
+      Setting.remotestorage_enabled = true
+    end
+
+    it "contains all enabled services" do
+      expect(Setting.available_services).to eq(%w[
+        discourse ejabberd mastodon remotestorage
+      ])
+    end
+  end
+
+end