Resolve "use `@` in hotline messages and replies"

Closes #344 (closed)

This PR:

• removes validation for a pound sign before a messageId in a reply
• adds @ as an alias for REPLY
• removes # in all help text and replaces with @ when appropriate
• modifies tests to make sure this is backwards compatible

@aguestuser @mari I have not staged this yet by trying it out locally, but would love a review to make sure I'm headed in the right direction.