Add support for using custom email message when creating a query

Fix #206.

Signed-off-by: Don Naegely <naegelyd@gmail.com>
Looking at the email title vs. the body, I think the "The query ... has been shared with.." is redundant. The body should always include a footer "View the query on {site_site}" (like github emails). If a message is present, simple add a horizontal rule (hyphen) between the message and footer (again like github).