webapp: Fetch token balance from Drips smart contract

[nya-elimu]

Instead of simply displaying the current $ELIMU balance of a contributor's Ethereum account, display the collectable balance from @drips-network.

Change the link so that it points to https://www.drips.network/app/<eth_address> instead of https://etherscan.io.

---

• Related to https://github.com/elimu-ai/web3-token-api/issues/7
• Depends on https://github.com/elimu-ai/webapp/issues/1896

[nya-elimu]

Call the splittable function: https://etherscan.io/address/0xd0dd053392db676d57317cd4fe96fc2ccf42d0b4#readProxyContract#F29

To get the accountId of an Ethereum address, use BigNumber.from(address).