pkg/mailer/mail: render markdown in issue related emails (#5629)
pkg/mailer/mail: render markdown in issue related emails (#5629)

Reference #4552


mukaiu/gogs,mukaiu/gogs,gogits/gogs,gogits/gogs,gogits/gogs,mukaiu/gogs,chriswatt/gogs,chriswatt/gogs,chriswatt/gogs