Add the possibility to trigger pipelines for mirror updates to the mirrored repositories after the mirror is set up

Currently it is only possible to select the option Trigger pipelines for mirror updates upon setting up the mirror: https://docs.gitlab.com/ee/workflow/repository_mirroring/repository_mirroring_pull_settings.png

So if the users decide to use this option later on, when the mirror is already set up, they need to remove and then create the mirror once again.

Allow users to select/deselect option Trigger pipelines for mirror updates after the mirrored repo was set up