net-im/mastodon: Update to version 1.6.0
Upstream changes: https://github.com/tootsuite/mastodon/releases/tag/v1.6.0
Port changes: Mastodon's node modules and assets are now installed as part
of the package.
Reviewed by: mat, saper_saper.info
Differential Revision: https://reviews.freebsd.org/D12306