[31200] Allow deletion of colors

https://community.openproject.com/wp/31200

app/views/colors/_form.html.erb

@@ -29,7 +29,19 @@ See docs/COPYRIGHT.rdoc for more details.
 
 <%= error_messages_for 'color' %>
 
-<%= toolbar title: (color.new_record? ? t('timelines.new_color') : t('timelines.edit_color')) %>
+<%= toolbar title: (color.new_record? ? t('timelines.new_color') : t('timelines.edit_color')) do %>
+  <% if color.persisted? %>
+  <li class="toolbar-item">
+    <%= link_to color_path(color),
+                method: :delete,
+                data: { confirm: t('text_are_you_sure') },
+                class: "button -danger" do %>
+      <%= op_icon 'button--icon icon-delete' %>
+      <span class="button--text"><%= t :button_delete %></span>
+    <% end %>
+  </li>
+  <% end %>
+<% end %>
 
 <fieldset class="form--fieldset">
   <legend class="form--fieldset-legend"><%= t('timelines.properties') %></legend>