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).