Resolve body part URL handlers based on part mimetype
Summary:
This solves ambiguity of URL handlers where multiple URL handlers from
different plugins can handle the same URL or just handle URLs generically,
in which case the first plugin in the list wins regardless of if the
same formatter actually produced the body part.
Test Plan: Buttons in invitations work again
Reviewers: vkrause, knauss
Subscribers: KDE PIM
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D8353