Clean up `bp_get_group_avatar_thumb()` and `bp_get_group_avatar_mini()` functions:

* Actually allow usage of `$group` parameter if passed
* Use array over query string
* Add phpdoc blocks

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8752 cdf35c40-ae34-48e0-9cc9-0c9da1808c22