Allow territory founders to sell sub-territories

[huumn]

... and sub-territory founders to sell sub-sub-territories

• founder sets prices and billing is paid directly to them. Sub-territories must adhere at minimum to ancestor territory's economic rules. It's kind of like a soft fork - it can only get more restrictive and not less.
• we'll also probably want territory approval mechanisms so a founder can prevent sub-territories that they don't want to see founded

This should probably come after #712.

Backend changes expected:

• primary key on Sub will be an ltree such that a sub-territory in ~bitcoin called coinjoin has the primary key bitcoin.coinjoin