Added global variable to point to plugin website base url.
Added function to check if plugin add-ons are active.
Added function to form Google Analytics campaign URLs for better tracking.
Misc code cleanup.
Lots of admin sidebar changes:
Styles, copy, hide when add-ons are active, campaign urls.
Removed unused sc_decimal_to_stripe_amount() function.
Remove add-ons submenu for now.
Use global variable and GA campaign URL in settings help link.