Make repository settings for "Pull from a remote repository" a Vue application

Summary

Pull from a remote repository settings under repository settings started out as simple feature but quickly became complicated in a way it handles DOM for variety of cases. It is perfect candidate to be converted into Vue application as that will greatly reduce the complexity in a way we handle DOM manipulation.

Related Code

• HAML views are at app/views/projects/mirrors
• JS bundle at app/assets/javascripts/mirrors