Added styles for milestone date range in milestone list

app/assets/stylesheets/pages/milestone.scss

@@ -239,6 +239,12 @@
   }
 }
 
+.milestone-range {
+  color: $gl-text-color-tertiary;
+  margin: $gl-bar-padding 0;
+  font-size: $tooltip-font-size;
+}
+
 @include media-breakpoint-down(xs) {
   .milestone-banner-text,
   .milestone-banner-link {

app/views/shared/_milestone_expired.html.haml

 - if milestone.due_date || milestone.start_date
-  %div
+  .milestone-range
     = milestone_date_range(milestone)
 - if milestone.expired? and not milestone.closed?
   .status-box.status-box-expired Expired

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

@@ -29,7 +29,7 @@
         - if @project
           - if can?(current_user, :admin_milestone, milestone.project) and milestone.active?
             - if @project.group
-              %button.js-promote-project-milestone-button.btn.btn-transparent.btn-sm.btn-grouped.has-tooltip{ title: _('Promote to Group Milestone'),
+              %button.js-promote-project-milestone-button.btn.btn-blank.btn-sm.btn-grouped.has-tooltip{ title: _('Promote to Group Milestone'),
                 disabled: true,
                 type: 'button',
                 data: { url: promote_project_milestone_path(milestone.project, milestone),
@@ -38,7 +38,7 @@
                         target: '#promote-milestone-modal',
                         container: 'body',
                         toggle: 'modal' } }
-                = sprite_icon('level-up', size: 12)
+                = sprite_icon('level-up', size: 14)
 
             = link_to 'Close Milestone', project_milestone_path(@project, milestone, milestone: {state_event: :close }), method: :put, remote: true, class: "btn btn-sm btn-close btn-grouped"
           - unless milestone.active?