email changing: easier and explain the keybase.io email addresses

things as soon as keybase.io email addresses are working (not just m.keybase):
- fix the placeholder technique on the web email change form
- make the web email change form explain forwarding
- make the verification email - "activate @keybase.io and verify your account"
- make verification emails explain that it's necessary to click the link to get access you to your keybase.io address, and that this address will be your forwarding address

Locate the web email change form and the verification-email flow described in the issue. Review how the form currently handles placeholders and how the email explains activation and forwarding; done means the form is clearer and the verification message identifies the keybase.io address, required link click, and forwarding behavior.

Written by the indexing model from the issue text.