Update GrantRoundManager to inherit from Uniswap's SwapRouter, instead of call an external router

[mds1]

This saves gas because:

1. We no longer need an external call
2. We no longer need to transfer tokens to/from the Router, which will reduce costs significantly

[thelostone-mc]

This is done at the moment and we have implmented a donate method which leverages SwapRouter

[mds1]

I don't think this has been done? For #76 we're calling an external router, instead of making the GrantRoundManager itself a router

[mds1]

This has been converted into an issue here: #99