Rename "xmpp" user service back to "ejabberd"

If we ever add support for others, we can combine them as "xmpp" in
helper methods

app/controllers/services/chat_controller.rb

@@ -3,7 +3,7 @@ class Services::ChatController < Services::BaseController
   before_action :require_service_available
 
   def show
-    @service_enabled = current_user.service_enabled?(:xmpp)
+    @service_enabled = current_user.service_enabled?(:ejabberd)
   end
 
   private

app/jobs/xmpp_exchange_contacts_job.rb

@@ -2,8 +2,8 @@ class XmppExchangeContactsJob < ApplicationJob
   queue_as :default
 
   def perform(inviter, invitee)
-    return unless inviter.service_enabled?(:xmpp) &&
-                  invitee.service_enabled?(:xmpp) &&
+    return unless inviter.service_enabled?(:ejabberd) &&
+                  invitee.service_enabled?(:ejabberd) &&
                   inviter.preferences[:xmpp_exchange_contacts_with_invitees]
 
     ejabberd = EjabberdApiClient.new