add pre-save and post-save actions

@ddean4040 I've added a couple of actions (based on those in `BP_Groups_Group::save()`) to `BP_Groups_Hierarchy::save()` so my plugin can monitor any updates to group hierarchies. This saves checking every plugin that's compatible with bp-group-hierarchy for hierarchy changes. It also avoids the need to hook into the 'bp_group_hierarchy_subgroup_permission_options filter' to monitor changes made when submitting bp-group-hierarchy admin forms.

Cheers, Christian