[ticket/11587] Pass legend and teampage settings to group_create()

Without passing those values, the group is removed from the teampage and
the legend while submitting the edit form on the ucp groups manage page.

PHPBB3-11587
Merge branch 'develop' of https://github.com/phpbb/phpbb3 into ticket/11587
[ticket/11587] Add functional tests for group teampage settings

The group_legend and group_teampage settings, which are needed for the
teampage, are tested with these newly added functional tests.
Duplicate code has been reduced as much as possible.

PHPBB3-11587
Merge remote-tracking branch 'remotes/Marc/ticket/11587' into develop

# By Marc Alexander
# Via Marc Alexander
* remotes/Marc/ticket/11587:
  [ticket/11587] Add functional tests for group teampage settings
  [ticket/11587] Pass legend and teampage settings to group_create()