Refactor layout code

PageLayout has props

• mobileHeader: Optional
• leftNav: Required
• content: Required
• talkSidebar: optional

Components:

• MobileHeader: pass into mobileHeader
  • Has props: isTalk, title
• LeftNav: pass into leftNav
  • Contains left nav layout
  • Contains side drawer
• TalkSidebar: pass into talkSidebar
  • Contains right nav layout
  • Contains side drawer
• TalkDesktopHeader: part of content