Log Message:
matterircd: update to 0.28.0, make this a Go module

New features

- Add support for searching scrollback for specific posts/threads
- Allow ignoring Mattermost server version when checking if supported
- Support for mattermost version 9

Enhancement

- Include thread context for matterircd style hex numbers for replay and
  scrollback
- Emphasize system and reaction messages
- Use static initialisation for regular expressions to optimize
- Convert Mattermost markdown formatting / emphasis to IRC
- Update scrollback to support @@msgthreadID and the MM postlist URL

Bugfix

- Fix incorrect msg/thread counters