Improve create a plan UX

[Lpaydat]

Many people are baffled about how to create a default plan because the default one is simply an empty item, so many will think they must select a project from the list and are unaware that this field can be left blank.

Instead of an empty item, I believe we should use a placeholder with a different style (for example, Default Plan).

[heyacherry]

Thank you for your suggestions!
@lilian79511 please has a review on this feature.