Add option to remove confusing warning when using disable_custom_urls: true on login error

[turt2live]

For this server, I intentionally want to disable the custom URLs so people don't try and log in to matrix.org (and generally hide the complexity of Matrix from the less-technical users). It would be nice to disable this warning to avoid confusing people.

[jryans]

Huh, looks like the message was added a while ago for status.im. I agree that's confusing, since we're trying to simplify by locking down the HS, but now we're confusing people again with complexity.

[turt2live]

re-triaged to be lower because it really doesn't impact anyone.