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