Port template highlighting to KF5::SyntaxHighlighter

Summary:
Besides a better highlighting and more accurate spell-checking areas, this
also removes the last user of the old highlighting framework in KDE PIM.

Reviewers: #kde_pim, mlaurent

Reviewed By: #kde_pim, mlaurent

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D7674