Warn if a legacy addres is used

[gasull]

Legacy addresses are the source of confusion for many BCH users.

See Why you should never send your BCH to BTC address.

This changeset creates a popup warning window when the user tries to
send BCH to a legacy address. The window has a "Never show this again"
checkbox.

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This change is 

[cculianu]

This commit is a great idea. Thanks for the contribution. Will review...

[gasull]

I also wanted to point out that since Electron Cash and Electrum are very similar applications in look and feel, a user might have both Electron Cash and Electrum open at the same time, and copypaste a legacy address on Electron Cash when meaning to do it on Electrum. This changeset will mitigate those human errors.

[cculianu]

I also wanted to point out that since Electron Cash and Electrum are very similar applications in look and feel, a user might have both Electron Cash and Electrum open at the same time, and copypaste a legacy address on Electron Cash when meaning to do it on Electrum. This changeset will mitigate those human errors.

Hmm. Yeah I never thought about that.

[cculianu]

Ping @gasull

[gasull]

Ping @gasull

Sorry, I was unable to work on this lately. Getting back to it now.

[gasull]

@cculianu , this is ready to be code-reviewed again.

[cculianu]

Looks great. I like it. I reviewed. Merging. Note I want to modify the test a bit -- but I can do that myself. Just a word or two.

Great work. I like how you added a "Show details.." text.. and the text makes sense.