First pass at rearranging settings and forums settings screens to accommodate bbPress 2.x migration:

• Simplify and move old bb-config.php file write logic into Settings screen
• Introduce new functions for checking what possible mode the Forums screen is in
• Deprecate old forum admin code for now, will probably be removed later
• Deprecate $bp->bbconfig setting and use bp_get_option() instead
• Clean up the Settings screen and add dedicated groups for Profiles and Forums
• See #3598
• More to do