Reject Reason should not be editable when the share is already approved

[TejasRGitHub]

Is your idea related to a problem? Please describe.
While rejecting a share, the dataset owner / steward can add in a rejection reason.

When the dataset is approved, this field can still be updated and edited.

Describe the solution you'd like
After the dataset is approved, the reject reason field should not be editable

P.S. Don't attach files. Please, prefer add code snippets directly in the message body.

[SofiaSazonova]

It's covered in this PR