Show "merge immediately" dialog when MR's pipeline has not yet finished

What does this MR do?

In !20054 (merged), we added a warning dialog that is shown when a user selects the "merge immediately" merge train option.

However, this warning modal was only shown if the MR's pipeline had succeeded (i.e. the merge button read "Start merge train"). See #12679 (comment 257950857).

This MR updates the warning dialog's logic to also show if the user selects "merge immediately" before the MR's pipeline has finished (i.e. the merge button reads "Start merge train when pipeline succeeds").

GIF