Add some classes conditionally rather than using extend

This shrinks application.css from 2.4MB to 2.3
Using @extend with buttons meant there were 800 instances of
stat-text in the stylesheet, rather than just using the button
classes in haml.

I am not even sure under what circumstances the stat-text is
rendered, I tested it by hardcoding link_to_if to false

app/assets/stylesheets/pages/projects.scss

@@ -794,11 +794,6 @@
 }
 
 .project-buttons {
-  .stat-text {
-    @extend .btn;
-    @extend .btn-default;
-  }
-
   .nav > li:not(:last-child) {
     margin-right: $gl-padding-8;
   }

app/views/projects/_files.html.haml

@@ -14,7 +14,7 @@
 
   - if is_project_overview
     .project-buttons.gl-mb-3.js-show-on-project-root
-      = render 'stat_anchor_list', anchors: @project.statistics_buttons(show_auto_devops_callout: show_auto_devops_callout)
+      = render 'stat_anchor_list', anchors: @project.statistics_buttons(show_auto_devops_callout: show_auto_devops_callout), project_buttons: true
 
   #js-tree-list{ data: vue_file_list_data(project, ref) }
   - if can_edit_tree?

app/views/projects/_stat_anchor_list.html.haml

 - anchors = local_assigns.fetch(:anchors, [])
+- project_buttons = local_assigns.fetch(:project_buttons, false)
 
 - return unless anchors.any?
 %ul.nav
   - anchors.each do |anchor|
     %li.nav-item
       = link_to_if anchor.link, anchor.label, anchor.link, class: anchor.is_link ? 'nav-link stat-link d-flex align-items-center' : "nav-link btn btn-#{anchor.class_modifier || 'missing'} d-flex align-items-center" do
-        .stat-text.d-flex.align-items-center= anchor.label
+        .stat-text.d-flex.align-items-center{ class: ('btn btn-default' if project_buttons) }= anchor.label

app/views/projects/empty.html.haml

@@ -20,7 +20,7 @@
 
   .project-clone-holder.d-none.d-md-inline-block.mt-2.mr-2.float-left
     = render "projects/buttons/clone"
-  = render 'stat_anchor_list', anchors: @project.empty_repo_statistics_buttons
+  = render 'stat_anchor_list', anchors: @project.empty_repo_statistics_buttons, project_buttons: true
 
 - if can?(current_user, :push_code, @project)
   .empty-wrapper.gl-mt-7