Convert address inputs to lowercase to fix signature verification issue

[geovgy]

Stumbled upon a bug while trying to send POKT from my ledger device in which any capitalized letter in the "Send to Address" input field will spit back a signature verification error after signing the tx from ledger. This seems to relate to an older issue (#182 ). As a suggested fix for this, I converted the address values to lowercase strings.

cc: @kutoft

[height]

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