[IOS-861] Forces lowercase passphrase entry
[IOS-861] Forces lowercase passphrase entry

Previously the passphrase entry fields could accept uppercase
letters, resulting in a failed entry.

This lowercases the text as it's entered.

**To Test**

Try entering an uppercase letter into a passphrase field and verify
that it's converted to lowercase on the fly.

Fixes [IOS-861](https://breadwallet.atlassian.net/browse/IOS-861)

breadwallet/breadwallet-ios,breadwallet/breadwallet-ios,breadwallet/breadwallet-ios,breadwallet/breadwallet-ios,breadwallet/breadwallet-ios