Create mailers with `jetzig generate mailer <name>`. Mailers define
default values for email fields (e.g. subject, from address, etc.).
Mailers use Zmpl for rendering text/HTML parts.
Send an email from a request with `request.mail()`. Call
`deliver(.background, .{})` on the return value to use the built-in
mail job to send the email asynchronously.
Improve query/HTTP request body param parsing - unescape `+` and `%XX`
characters.
