Convert the frontend to use generic RPC_URL instead of INFURA_ID for ENS Resolution

[apbendi]

Currently, we use the INFURA_ID env var to get a provider in a number of places in the frontend. We want to move toward using a generic RPC_URL environment variable that can be used with any provider.

The INFURA_ID env var is currently used for a number of tasks, including ENS and CNS name resolution, checking for contract code, and as a fallback when the user's wallet provider rejects the transactions from accounts not associated with that user's wallet. It is used in both the frontend and the umbra-js subrepos.

We do not need to convert all of these over to a generic RPC_URL at once. For example, it would be useful to move in-app ENS Resolution only at first, so we could split the load across providers, and continue the refactor after that.