Revert "Frontend: Do not include files if plugins have been force disabled."

After discussing things over with @boonebgorges, we've decided that
commit c107038 is over-engineering a solution to something that is an
edge-case scenario.

Will add a simpler fix to address bbPress load issues in the next commit.

See #105.
Frontend: For bbPress, use 'bbp_activation' for the plugin check instead of 'bbpress'.

Currently, CBOX network-activates all of its bundled plugins including
bbPress.  This has led to scenarios where developers have tried to disable
bbPress on sub-sites, while keeping CBOX's code intact.

One of those scenarios involves turning on the "late load" feature of
bbPress and then removing the bbPress 'plugins_loaded' hook from loading:
http://commonsinabox.org/groups/help-support/forum/topic/scope-of-bbpress-plugin-and-associated-issues/#post-4680

Because the bbpress() function always exists in this scenario, we need to
change CBOX's function check to something that is loaded a little later
such as bbp_activation().  This will ensure that the "late load" method
will work as expected.

In a future release, CBOX will have a new tier so certain plugins like
bbPress are only activated on the main BuddyPress root blog to prevent
all these workarounds :)

Fixes #103, #105.