Improve custom domain setup/docs

The way that this currently works doesn't inspire confidence in mailbox. Really fundamental mistakes.

1. On the setup screen, for some reason, the domain concatenated without a dot but with a dot at end.

2. In the docs, there are dots at the end of mail server address. (Admittedly, this one is easy for anyone to realize that it's false but... that's kind of the point. Non-technical people shouldn't write technical docs.)