Ensure bp_get_user_has_avatar() returns false when the default avatar is returned

Necessitated by changes in return values introduced in r7769

See #5380

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git-svn-id: https://buddypress.svn.wordpress.org/trunk@8243 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Allow bp_get_group_has_avatar() to accept a group_id param

Also fix some logic that was causing the function to return true when the
group avatar matched the default avatar (meaning that no local avatar had been
uploaded). See r7769

Fixes #5380

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git-svn-id: https://buddypress.svn.wordpress.org/trunk@8244 cdf35c40-ae34-48e0-9cc9-0c9da1808c22