handle underscore not being allowed in dns names

[pbiggar]

Trying to access domain names with underscores in the subdomain doesn't work with the .NET httpclient. It's marked as WONTFIX: dotnet/runtime#35880. There is a workaround but I'd rather not accrue the debt considering it's technically wrong.

This is a problem because some of our users have usernames with underscores. So when they access user_name.builtwithdark.com with our httpclient, there's an SSL error:

<ErrorRail: Error "The SSL connection could not be established, see inner exception. The remote certificate is invalid according to the validation procedure: RemoteCertificateNameMismatch">

The correct solution, according to the grand plan of dark that's in my head, is to not use usernames in subdomains. I'll write up an issue for that. In the meantime, we need to patch over this in a forward compatible way.

• disallow underscores in usernames in the adduser canvas for signup
• disallow in Accounts.fs when creating users (note the UserName.create regex will still need to support users with underscores so we can't change that)
• tell users about the subdomain situation, offer to change their usernames
  • we would need to rename the user, the canvas, and the custom_domain hosts
  • update signup page
• re-allow underscores when switch to darklang.io is complete

[pbiggar]

[StachuDotNet]

re-allow underscores when switch to darklang.io is complete

relevant issue: #3824.