Enable the Check Before Send plugin by default

Summary:
This is one of KMail's "killer" features, so we should have it enabled by
default to make it easier for users to discover it and use it. It's very
popular.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D14887