Use "mail-mark-important" icon consistently

Summary:
KMail already uses this icon in it's menu and context menu. For other statuses
we already use mail-* icons too. I think this also more correct since according
to the standard the emblem icons are "Icons for tags and properties of files,
that are displayed in the file manager".
https://specifications.freedesktop.org/icon-naming-spec/icon-naming-spec-latest.html#context

Reviewers: KDE PIM, mlaurent, VDG

Reviewed By: KDE PIM, mlaurent

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D27507