Add proper explanation to MR in archived projects

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/views/projects/merge_requests/show/_mr_accept.html.haml

 - unless @allowed_to_merge
-  .bs-callout
-    %strong You don't have permission to merge this MR
+  - if @project.archived?
+    .bs-callout.bs-callout-warning
+      %strong Archived projects cannot be committed to!
+  - else
+    .bs-callout
+      %strong You don't have permission to merge this MR
 
 
 - if @show_merge_controls

app/views/projects/show.html.haml

@@ -9,8 +9,8 @@
   .col-md-3.project-side.hidden-sm
     .clearfix
       - if @project.archived?
-        .alert
-          %h5
+        .alert.alert-warning
+          %h4
             %i.icon-warning-sign
             Archived project!
           %p Repository is read-only