zkSync updates part 2

[mds1]

Second round of zkSync changes is ready for review. There are a lot of code paths and I've only tested against Rinkeby, so there are almost guaranteed to be some bugs and issues so please report any issues you find while testing this PR.

Changes in this PR are explained below.

Background

There are two zkSync accounts to consider:

1. gitcoinSyncWallet — A zkSync wallet generated by having the user sign a Gitcoin-specific message. This gives us the private key used to create this deterministic wallet. Since we have the private key, we can send transfers without prompting the user
2. nominalSyncWallet — The default zkSync wallet generated through the zkSync SDK, using the user's web3 account. We do not have the private key, so the user receives a signature prompt on each transfer

User Flows

Upon clicking "Checkout with zkSync", the flow looks like this:

1. Does user have sufficient balance in nominalSyncWallet to cover all deposits? If yes, we call this Flow A which only requires signatures and is extremely cheap and fast. If no, we call this Flow B, which requires an L1 transaction so is costly and slow.

2. Flow A:

   1. Prompt user to sign in to gitcoinSyncWallet (user sees prompt 1)
   2. Prompt user to sign in to nominalSyncWallet (user sees prompt 2)
   3. If user has never used zkSync (e.g. maybe someone sent them funds and they never used it), they see a signature prompt to register their zkSync account (user prompt 3)
   4. Prompt user to transfer funds from nominalSyncWallet to gitcoinSyncWallet (user sees prompts 3 or 4–n, where n = number of different currencies selected in cart)

3. Flow B:

   1. "Advanced settings" toggle lets users choose to deposit additional funds on top of their donation
   2. Prompt user to sign in to gitcoinSyncWallet (user sees prompt 1)
   3. Prompt user to approve ERC20 tokens (user sees prompts 2–n, where n depends on number of different currencies selected in cart and pre-existing allowances)
   4. Prompt user to deposit funds into zkSync (user sees prompt n+1)

4. Flows A and B now converge

5. We sign each donation transfer with the gitcoinSyncWallet (no user prompts)

6. We dispatch each donation transfer from the gitcoinSyncWallet (no user prompts)

7. If there are any excess funds left in gitcoinSyncWallet, we transfer them to nominalSyncWallet. (Sometimes there will be dust leftover in gitcoinSyncWallet based on amount packing and fees, but there's not much we can do about this).

Notes

The main remaining tasks for zkSync that are not included in this PR are:

• Transaction validator must be updated to support Flow A
• Make gas estimates smarter — for now it's temporarily hardcoded at 1M
• Make suggestion to user whether to use zkSync vs. L1 depending on balances and estimated gas costs. Our suggestion here cannot be perfect and will be wrong sometimes. This is because we'll only have imperfect gas estimates, but we may be able to tweak this as the round goes on and we get some real data

Also, apologies for the monster that cart.js has become 😅

cc @thelostone-mc @octavioamu @owocki @apbendi