Add expiration time to PartyBid

[msfeldstein]

Adds an expiration timer to PartyBid so that a party that never meets reserve can get its funds back after a pre-determined time.

• When starting a Party, the creator must set an expiration datetime
• Before that timestamp, people can contribute and place bids as normal
• If the reserve is met and the auction starts before the expiration time, it can never be expired, and things proceed as normal until the auction ends
• If the expiration passes before the reserve is met anyone in the party can call expire, and then no new contributions can be made, a bid can't be placed (although that shouldn't be possible anyway?) and everyone can withdraw their contributions

The party can choose to not call expire and continue trying to collect contributions and meet the reserve (at which time expire can no longer be called)

[msfeldstein]

I can move expiresAt into Party.sol, but should we merge the two repos so we don't have diverging base contracts?

[msfeldstein]

The shared contract did feel more complicated to me i wasn't sure what the purpose of it was (especially since partybuy lives elsewhere)