Modals, OnClickOutside and ScrollLock

What this PR addresses

• Modals jump to top of viewport
• IE checkout is broken
• Adds OnClickOutside to modals
• Modals cause content flicker (minor issue)

Why?

react-scrolllock

How this PR fixes things

Replace react-scrolllock with body-scroll-lock

What this PR impacts

• Checkout
• Header
• Tour pages
• Wishlist
• Trip-builder

If this goes live, it will need lots of testing