Fix fatal error on front end when bbPress disabled on subsites

When bbPress is force-disabled on sub-sites, commons-in-a-box throws a fatal error. Same issue as #100 essentially, because in some cases people do not want bbPress active on sites other than the main site. See, for example [this discussion](http://commonsinabox.org/groups/help-support/forum/topic/scope-of-bbpress-plugin-and-associated-issues/).