Merge branch 'milestone_start_date' into 'master'

app/assets/javascripts/due_date_select.js.es6

app/assets/stylesheets/framework/issue_box.scss

app/controllers/groups/milestones_controller.rb

app/controllers/projects/milestones_controller.rb

app/helpers/issues_helper.rb

app/helpers/milestones_helper.rb

app/models/concerns/milestoneish.rb

app/models/global_milestone.rb

app/models/milestone.rb

app/views/groups/milestones/new.html.haml

app/views/projects/milestones/_form.html.haml

app/views/projects/milestones/show.html.haml

app/views/shared/_milestone_expired.html.haml

app/views/shared/milestones/_form_dates.html.haml

app/views/shared/milestones/_top.html.haml

changelogs/unreleased/milestone_start_date.yml

db/migrate/20161115173905_add_start_date_to_milestones.rb

db/schema.rb

doc/api/milestones.md

lib/api/entities.rb