Checking balance should not require hex prefix

from @flyswatter.

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Right now entering a non-hex-prefixed address into the check balance field will silently fail to the user.

It would be easy enough to call `ethUtil.addHexPrefix(userInput)`, so I think that would be better.

Start by locating the check balance field and its address-handling entry point, then inspect where ethUtil.addHexPrefix can be applied to the user input. Verify that a non-hex-prefixed address checks the balance successfully instead of failing silently.

Written by the indexing model from the issue text.