Rename "xmpp" user service back to "ejabberd"

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

spec/requests/webfinger_spec.rb

@@ -44,7 +44,7 @@ RSpec.describe "WebFinger", type: :request do
         before do
           allow_any_instance_of(User).to receive(:ldap_entry).and_return({
             uid: user.cn, ou: user.ou, mail: user.email, admin: nil,
-            services_enabled: ["xmpp"]
+            services_enabled: ["ejabberd"]
           })
         end
 
@@ -100,7 +100,7 @@ RSpec.describe "WebFinger", type: :request do
         before do
           allow_any_instance_of(User).to receive(:ldap_entry).and_return({
             uid: user.cn, ou: user.ou, mail: user.email, admin: nil,
-            services_enabled: ["xmpp"]
+            services_enabled: ["ejabberd"]
           })
         end