Refactor settings model

Move the various sections to their own concerns, so they're easier to
find and maintain

app/models/concerns/settings/ejabberd_settings.rb

@@ -0,0 +1,19 @@
+module Settings
+  module EjabberdSettings
+    extend ActiveSupport::Concern
+
+    included do
+      field :ejabberd_enabled, type: :boolean,
+        default: ENV["EJABBERD_API_URL"].present?
+
+      field :ejabberd_api_url, type: :string,
+        default: ENV["EJABBERD_API_URL"].presence
+
+      field :ejabberd_admin_url, type: :string,
+        default: ENV["EJABBERD_ADMIN_URL"].presence
+
+      field :ejabberd_buddy_roster, type: :string,
+        default: "Buddies"
+    end
+  end
+end