bp_get_activity_avatar applies incorrect filter

[agentmaximus]

The function bp_get_activity_avatar in bp-activity/bp-activity-templatetags.php applies the filter bp_get_group_avatar instead of bp_get_activity_avatar

It is not clear whether this is the intended implementation.

It would also be useful to add a filter inside bp_get_activity avatar for overriding the avatar default dimensions. The default dimensions are used inside the sitewide activity widget through the call to bp_get_avatar(). It would be useful for theme developers to be able to override these defaults (currently 20px x 20px).

[johnjamesjacoby]

I'm going to audit all the avatar functions and try to make them more flexible, to accommodate for using all of their $args.

[johnjamesjacoby]

(In [2071]) Fixes #1291 (props agentmaximus)