After this diff, bch cash addresses are no longer supported in the "Pay to" field, in BIP21 URIs or any method of entering an address that uses `Address.from_string` without setting `support_arbitrary_prefix=True`.
The address converter in the GUI still support any prefix. This diff also makes the command line / RPC `addressconvert` command support arbitrary prefixes.
But now we no longer support bch addresses without an explicit prefix.
This diff also incidentally fixes support for "ectest:..." URIs when running in testnet mode. Previously `web.parseable_schemes` did not correctly account for the net parameter, and always returned `("ecash", "bitcoincash")`
This is a follow-up to https://github.com/Bitcoin-ABC/ElectrumABC/pull/250 (dropping of display of BCH addresses in the UI).