Warn in comment field if task is closed as duplicate

Summary:
Display a placeholder text in the text comment field of a Maniphest task if the task status has been set to Duplicate.
This makes it clearer to users (who may have not checked the task status at the top of the page) not to fragment conversations.

Closes T15749

Test Plan:

• Be logged in and go to a task which is closed as a duplicate and see the placeholder text in the field to add a comment.
• Be logged in and go to tasks which are not closed as a duplicate and see no placeholder text in the field to add a comment.
• Go to other places whose code calls a PhabricatorApplicationTransactionCommentView constructor and check that it still renders correctly, for example Ponder in http://phorge.localhost/Q1, Slowvote in http://phorge.localhost/V1, Differential in http://phorge.localhost/D1

Reviewers: O1 Blessed Committers, valerio.bozzolan, avivey

Reviewed By: O1 Blessed Committers, valerio.bozzolan, avivey

Subscribers: avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15749

Differential Revision: https://we.phorge.it/D25546