Move SEND_CONFIRM_DELAY to sendcoinsdialog.h

Summary:
This will help reduce merge conflicts for future backports.
This change is a partial backport of RBF PR 9697
https://github.com/bitcoin/bitcoin/pull/9697/files

Test Plan: ninja

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2272