Center everything on mobile instead and correct btn-link changes

app/assets/stylesheets/framework/buttons.scss

@@ -423,8 +423,6 @@
 }
 
 .btn-link {
-  padding: 0;
-
   &.btn-secondary-hover-link {
     color: $gl-text-color-secondary;
 

app/assets/stylesheets/pages/milestone.scss

@@ -218,10 +218,15 @@
     position: relative;
   }
 
+  .btn-link {
+    padding: 0;
+  }
+
   @media (max-width: $screen-xs-max) {
+    text-align: center;
+
     .illustration {
       display: block;
-      text-align: center;
       margin: $gl-padding 0;
     }
   }

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

@@ -5,7 +5,7 @@
     %p.append-bottom-default.prepend-top-5
       Use group milestones to manage issues from multiple projects in the same milestone.
       %br
-      = button_tag 'Promote these project milestones into a group milestone.', class: 'btn-blank'
+      = button_tag 'Promote these project milestones into a group milestone.', class: 'btn-link'
 
     .popup.hide
       .body