Create Issue modal should disable labels etc instead of hiding the elements

[mickmister]

Before selecting a repo, there are 3 total fields available. Once the repo is selected, we place labels, assignees, and milestones in the middle of the form. When a repo is selected that does not have labels etc, the form elements disappear.

We can make the modal experience more fluid by always showing labels, assignees, and milestones, but disable the elements when the repo does not support those fields.

[sanjaydemansol]

@mickmister please assign this to me.