Improve editing messages with link previews.

Now preview state can be one of (allowed, cancelled, empty-in-edit).

In case of editing a message without preview we set the state to
empty-in-edit and it changes to allowed if the links in the message
are changed somehow.

That way we don't need to cancel the preview when editing a message
with a cancelled preview and at the same time adding a link to
a message that had no preview in the first place will add a preview.

Telegram/SourceFiles/support/support_helper.cpp

@@ -155,7 +155,7 @@ Data::Draft OccupiedDraft(const QString &normalizedName) {
 			+ normalizedName },
 		MsgId(0),
 		MessageCursor(),
-		false
+		Data::PreviewState::Allowed
 	};
 }