feat: contract code to mint tickets

[LuqiPan]

My first stab at updating the contract to mint tickets, notably, update contract terms to accept inventory then check want against inventory to make sure we have enough inventory and check give against total price to make sure user offers enough tokens.

This is still pretty rough so any and all feedbacks are welcomed.

TODOs:

• [x] make the last test case in test-contract.js pass, it's using mockBootstrap and I don't know quite understand how it works
• [x] update comments and JSDocs accordingly

[dckc]

update UI code and tests accordingly

updating the UI code seems to fit more comfortably in a separate PR

[aj-agoric]

Marking this PR as done in the project board since this is merged.