Don't allow welcome mail to be sent to users who can't login

Summary:
Fixes T9446. We allow administrators to send "Welcome" mail to bots and mailing lists.

This is harmless (these links do not function), but confusing.

Instead, disable this option in the UI and explain why it is disabled when it is clicked. Also prevent generation of this mail lower in the stack.

Test Plan:

• Viewed a bot page, saw action disabled, clicked it, got explanation.
• Viewed a normal user page, saw action enabled, clicked it, sent welcome email.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9446

Differential Revision: https://secure.phabricator.com/D14134