Offer management

[carlsverre]

• [ ] add offer table view to Admin page
• [ ] new component: offer creation form
  • draw offer polygon on a map (or select plus code polygon to use)
  • should support modifying an existing offer or defining a new one
  • inline segment creation flow
    • show number of matching subscribers in real time (inspired by google/facebook ads creation flow)
• [ ] new component: offer creation modal (embeds offer creation form)
• [ ] add offer creation to the overview page to help explain how offers work
• [ ] add offer creation to admin page as part of the offer table