Fetch BrightIdUserRegistry context from contract

[pettinarip]

ISSUE MIGRATED FROM https://github.com/ethereum/clrfund/issues/283

Noticed we have an env variable VUE_APP_BRIGHTID_CONTEXT.

Might be easier to maintain if we just fetch that via the context() getter on the BrightIdUserRegistry contract (it's a public storage variable): https://github.com/ethereum/clrfund/blob/develop/contracts/contracts/userRegistry/BrightIdUserRegistry.sol#L14

Not a HUGE win, so let me know what you folks think. I don't think we currently fetch any data about the user registry to keep in the store, so maybe it is easier having as an ENV variable 🤷