fixing Group#extensive_name getter.

was broken in https://github.com/fiedl/wingolfsplattform/commit/9d3729e431d6ae1e28316a7edca5f2a6ccf4aa95.
implementing ops announcements.

For an announcement, write markdown to the file tmp/announcement.md.
dag links: use fork without the uniqueness validation of dag links.

this way, we can have a user leave a group and later rejoin it.

former, abandoned commits:
* ad9d7b2
* b14b40e
* 18f8ae0

See: https://github.com/fiedl/wingolfsplattform/commit/18f8ae07716f8607acc3f4fbb37da35f56c58dd4

Trello: https://trello.com/c/84R7LS8g/564-multiple-daglinks-erlauben