Add tab-specific body classes for group creation and group admin

[boonebgorges]

We currently add the body class group-create to group creation steps and group-admin to group admin tabs. Since we also have information available about the name of the specific steps (as they're registered in BP_Group_Extension, for instance) we should be adding them to the body class as well, for the benefit of themers.

[boonebgorges]

(In [5941]) Introduces template function bp_get_groups_current_create_step() to avoid direct calls to the bp global.
Replaces calls to bp->groups->current_create_step with bp_get_groups_current_create_step() throughout BP.
Adds current create step to body classes during group creation process. See #4095

[boonebgorges]

(In [5942]) Adds template tag bp_get_group_current_admin_tab() and implements throughout BuddyPress for accessing current group admin tab data.
Adds current admin tab slug to the body class in the group admin section.
Fixes #4095