Multiselect dropdown replacement for territory crossposting

[huumn]

For the UI, We'll probably want something like this: https://react-select.com/home

On the backend:

• we'll need a many-to-many relationship between Item and Sub
• to avoid additional joins every time we load an Item, we'll want to denormalize all the territories Item belongs to in trigger that populates an array on Item called subNames
• create_item and update_item stored procedures will need to be updated

Economics:

• Posting in multiple territories is cumulative
  • e.g. if posting in ~bitcoin and ~nostr and each costs 10 sats to post, the resulting fee is 10+10=20 sats.