Tweak remaining CSS for Markdown preview

Missed a few items in !1378

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

@@ -21,7 +21,7 @@
       .form-group.milestone-description
         = f.label :description, "Description", class: "control-label"
         .col-sm-10
-          = render layout: 'projects/md_preview', locals: { preview_class: "wiki" } do
+          = render layout: 'projects/md_preview', locals: { preview_class: "md-preview" } do
             = render 'projects/zen', f: f, attr: :description, classes: 'description form-control'
             .hint
               .pull-left Milestones are parsed with #{link_to "GitLab Flavored Markdown", help_page_path("markdown", "markdown"), target: '_blank'}.

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

@@ -21,7 +21,7 @@
   .form-group.wiki-content
     = f.label :content, class: 'control-label'
     .col-sm-10
-      = render layout: 'projects/md_preview', locals: { preview_class: "wiki" } do
+      = render layout: 'projects/md_preview', locals: { preview_class: "md-preview" } do
         = render 'projects/zen', f: f, attr: :content, classes: 'description form-control'
         .col-sm-12.hint
           .pull-left Wiki content is parsed with #{link_to "GitLab Flavored Markdown", help_page_path("markdown", "markdown"), target: '_blank'}

app/views/shared/issuable/_form.html.haml

@@ -24,7 +24,7 @@
   = f.label :description, 'Description', class: 'control-label'
   .col-sm-10
-    = render layout: 'projects/md_preview', locals: { preview_class: "wiki", referenced_users: true } do
+    = render layout: 'projects/md_preview', locals: { preview_class: "md-preview", referenced_users: true } do
       = render 'projects/zen', f: f, attr: :description,
                                classes: 'description form-control'
       .col-sm-12.hint