make projects dropdown accessible

app/assets/stylesheets/framework/header.scss

@@ -160,11 +160,15 @@ header {
       }
 
       .dropdown-toggle-caret {
+        color: $gl-text-color;
+        border: transparent;
+        background: transparent;
         position: relative;
         top: -2px;
         width: 12px;
         line-height: 12px;
         margin-left: 5px;
+        padding: 0;
         font-size: 10px;
         text-align: center;
         cursor: pointer;

app/helpers/projects_helper.rb

@@ -61,7 +61,9 @@ module ProjectsHelper
     project_link = link_to simple_sanitize(project.name), project_path(project), { class: "project-item-select-holder" }
 
     if current_user
-      project_link << icon("chevron-down", class: "dropdown-toggle-caret js-projects-dropdown-toggle", aria: { label: "Toggle switch project dropdown" }, data: { target: ".js-dropdown-menu-projects", toggle: "dropdown" })
+      project_link << button_tag(type: 'button', class: "dropdown-toggle-caret js-projects-dropdown-toggle", aria: { label: "Toggle switch project dropdown" }, data: { target: ".js-dropdown-menu-projects", toggle: "dropdown" }) do
+        icon("chevron-down")
+      end
     end
 
     full_title = "#{namespace_link} / #{project_link}".html_safe