Pass more arguments through bp_get_message_thread_avatar()

Allowing more bp_core_fetch_avatar() arguments gives greater flexibility to
themers and plugin authors.

Fixes #4959

Props henrywright

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7435 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Don't use extract() in bp_get_message_thread_avatar()

Just don't do it, dude.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7436 cdf35c40-ae34-48e0-9cc9-0c9da1808c22