resolve review comments:

- use .mx-auto for avatar
- remove parent div for group_icon

app/views/projects/forks/_fork_button.html.haml

@@ -5,9 +5,9 @@
   .bordered-box.fork-thumbnail.text-center.prepend-left-default.append-right-default.prepend-top-default.append-bottom-default.forked
     = link_to project_path(forked_project) do
       - if /no_((\w*)_)*avatar/.match(avatar)
-        = group_icon(namespace, class: "avatar rect-avatar s100 identicon")
+        = group_icon(namespace, class: "avatar rect-avatar s100 identicon mx-auto")
       - else
-        .avatar-container.s100
+        .avatar-container.s100.mx-auto
           = image_tag(avatar, class: "avatar s100")
       %h5.prepend-top-default
         = namespace.human_name
@@ -18,9 +18,9 @@
         class: ("disabled has-tooltip" unless can_create_project),
         title: (_('You have reached your project limit') unless can_create_project) do
       - if /no_((\w*)_)*avatar/.match(avatar)
-        = group_icon(namespace, class: "avatar rect-avatar s100 identicon")
+        = group_icon(namespace, class: "avatar rect-avatar s100 identicon mx-auto")
       - else
-        .avatar-container.s100
+        .avatar-container.s100.mx-auto
           = image_tag(avatar, class: "avatar s100")
       %h5.prepend-top-default
         = namespace.human_name