Icon accessibility [belongs to #5423]

app/views/projects/settings/_versions.html.erb

@@ -158,7 +158,7 @@ See doc/COPYRIGHT.rdoc for more details.
                             { class: 'button -alt-highlight',
                               aria: {label: t(:label_version_new)},
                               title: t(:label_version_new)}) do %>
-        <i class="button--icon icon-add"></i>
+        <%= op_icon('button--icon icon-add') %>
         <span class="button--text"><%= l(:label_version) %></span>
       <% end %>
     </div>

lib/open_project/backlogs/engine.rb

@@ -108,7 +108,7 @@ module OpenProject::Backlogs
            before: :calendar,
            param: :project_id,
            if: proc { not(User.current.respond_to?(:impaired?) and User.current.impaired?) },
-           html: { class: 'icon2 icon-backlogs' }
+           icon: 'icon2 icon-backlogs'
     end
 
     assets %w(