Send email notification when the milestone is set or changed with mass assignment

Dev: https://dev.gitlab.org/gitlab/gitlabhq/issues/1806

cc/ @sytses @dzaporozhets

Title changed from Send email notification when the milestone is set or changed to Send email notification when the milestone is set or changed with mass assignment

Milestone changed to 8.5

/cc @JobV

Already fixed by https://gitlab.com/gitlab-org/gitlab-ce/commit/b673d87227867f6d142ee6e615c750a600661c6b :)

Status changed to closed