databases/mantis: Update to 2.16.0

• Use upstream release tarball instead of doing our own vendoring
• Add missing PHP extensions
• Flavorize
• Update plugins and install them by default
• Add LICENSE
• Improve pkg-descr

Changes: https://mantisbt.org/bugs/changelog_page.php?project=mantisbt&version=2.16.0
PR: 229880
Reviewed by: dvl, mat, ndowens@yahoo.com
MFH: 2018Q3 (with r477954, r477984)
Security: 0822a4cf-9318-11e8-8d88-00e04c1ea73d
Differential Revision: https://reviews.freebsd.org/D16890