Expose MessagePart directly to the attachment Grantlee templates

Summary:
This reduces code duplication and makes this easier to re-use for plugins.
Additionally, it fixes two issues with attachment rendering:

• for many attachments we weren't showing links, making it unnecessarily hard to save or open them
• we were double-escaping attachment labels and descriptions

Reviewers: knauss

Reviewed By: knauss

Subscribers: KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D8124