Fix missing alt attribute in phases images

An image without an alt text is invalid HTML and is confusing for screen
reader users.

This is just a quick patch which partially solves this problem. The
image doesn't necessarily need to be a decorative one, so administrators
should have the option to provide an alternative text for the images.

app/components/budgets/phases_component.html.erb

@@ -55,7 +55,7 @@
 
           <% if phase.image.present? %>
             <div class="budget-phase-image">
-              <%= image_tag phase.image.attachment.url(:large) %>
+              <%= image_tag phase.image.attachment.url(:large), alt: "" %>
             </div>
           <% end %>
         </div>