Improvement on token swaps (feature request)

[kopeboy]

Allow fixed price repeatable swaps with a max amount & duration:

The DAO proposal can set:

1. token A to sell from the treasury
2. token B to accept from any address
3. NEW: max amount of A that can be sold (to be immediatly available & locked)
4. NEW: fixed exchange rate
5. NEW: block/time deadline for the swap offer validity

More than a swap between two specific DAOs (how it's currently phrased in the UI) this becomes an offer to the public and a nice way to fund startups (similar to "Series A/B/C funding" if you swap USDC for the DAO token, but there could be different pairs & use cases).