Ensure bad checksum transactions are blocked

[almindor]

Add a opt-out setting that needs to be enabled (with a warning check pass) to allow bad checksum sends to work.

Now that EIP55 is finalized and supported by Etherwall and most others there should be no reason to allow bad checksum-ed addresses to be used. Example in #27

If users still need to do so they'll need to go to settings and specifically allow this operation in general before proceeding.

Transaction dialog should show this as an error otherwise and block the user from sending their ETH out.

[almindor]

Decided to not allow bad checksums at all and skip the complication of setting.

People can use etherscan to "double check" their destinations and get the properly CRCed addresses.