Compile a rule description

Turns a plain-English description of which received emails to match into a rule tree over the published column contract, with a plain-English explanation of what it matches, the assumptions made, and warnings for anything the columns cannot express. Where the columns fall short the rule may carry up to three yes/no questions, asked about an email only after its conditions match. Nothing is saved. The SQL the rule renders to is returned only when include_sql is true. Needs mail:read, a credential acting as a member with their full access (API keys, agent keys and narrowed tokens are refused), and the email rules feature on the organization.