models/issue_mail: add assignee to issue related emails (#5628)
models/issue_mail: add assignee to issue related emails (#5628)

Reference #4220
mukaiu/gogs,chriswatt/gogs,gogits/gogs,mukaiu/gogs,chriswatt/gogs,chriswatt/gogs,mukaiu/gogs,gogits/gogs,gogits/gogs