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Bumps stable tag to 1.8.1
Make sure groupblog ID exists before trying to redirect.

Basically reapplies changes from commit 45c91dc / 55fb215 due to merge
conflict from commit fa02cb2.
Remove unused $bp global in bp_groupblog_get_user_role().
Fix blog roles on group ban and removal.

This commit adds back the 'ban' and 'remove' actions and drops the member
to subscriber for the groupblog in question.

Missed during a merge conflict stemming from commit f9d4824.
Fix groupblog redirect problem once more.

Thanks to mrjarbenne for reporting and testing.