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Consolidate instances of cuny_buddypress_group_actions()

Group sidebar navigation comes from the function cuny_buddypress_group_actions(),
which is called directly in the template files. Previously, this function had
been defined inline in various template files, with the result that markup was
slightly different when looking at pages that used plugins.php vs home.php to
render. This changeset sets up a canonical version of the function, and removes
all others.

See #934
Pinking shears