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