Revert "Merge pull request #105 from christianwach/bbpress-fix"

This reverts commit 19fc8c7dcd61b38935ff733578e4dd1712d5dca4, reversing
changes made to 752e5ca07e67f2b5c2bfd090f89ebefcf11fd6e1.
Frontend: Do not include files if plugins have been force disabled.

A plugin can be force disabled if a code snippet removes the plugin's
main code from loading.

For example, bbPress can be disabled on sub-sites using this code snippet:
http://commonsinabox.org/groups/help-support/forum/topic/scope-of-bbpress-plugin-and-associated-issues/#post-4680

Previously, our frontend files were being included without doing proper
checks to see if plugins were disabled or not.  This commit changes the
logic so we do proper checks to see if a plugin is enabled before
including our files and hooks.

Currently, CBOX network-activates all of its bundled plugins.  In a future
release, we will use a new tier so certain plugins like bbPress are only
activated on the main BuddyPress root blog.

Fixes #100, #105.
Frontend: Remove bbp_get_version() function exists call.

This shouldn't be necessary any more.

See commit c107038.