Create Subgraph for Incentives

[omarish]

Once this launches, one big question will be: where are all the incentives? We
should make them easily discoverable.

I propose we do this by either:

1. Creating a new subgraph on TheGraph
2. Adding to the existing Uniswap Subgraph

Regardless of how we do it, I think we'll want to do the following:

(Writing this from my iPad and writing event names from memory, will take a look when I'm back in front of my computer to make sure I got the event names right.)

I think the simplest useful thing we could build here is something that listens for the IncentiveCreated and IncentiveEnded events coming from the Staker contract. With this, we should be able to build a subgraph that can be queried to show all active and ended incentives.

Once we have this, we should also figure out how much reward balance remains for each incentive.

One complication: we have not yet deployed the Staker contract, so whomever takes this issue might need to use the http://github.com/omarish/uniswap-V3-devtools repo to test locally, or to wait until the contract is deployed on testnet/mainnet.

[kelonye]

Here's an example subgraph uniswap-v3-staker-subgraph, that is used to power uniswap-v3-staker-frontend.

[uniearn]

We created one here Uniswap-V3-Staker-Subgraph. We are using it for our fair launch of UFI, a YFI-like yield aggregator on Uniswap V3.
The subgraph is deployed at Uniearn Subgraph.