Fix #4990: Allow admins to delete uploads.

Allow admins to delete media asset files.

This only deletes the image file itself, not the upload or media asset record. The upload will still
be in the user's upload list, but the image will be gone. The media asset page will still exist, but
it will only show the file's metadata, not the image itself. We don't delete the metadata so we have
a record of what the file's MD5 was and who uploaded it, to prevent the file from being uploaded
again and to take action against the user if necessary.

app/views/media_assets/destroy.js.erb

@@ -0,0 +1 @@
+location.reload();

app/views/media_assets/show.html.erb

@@ -73,6 +73,18 @@
                         <%= search_icon %> <%= Danbooru.config.app_name %>
                       <% end %>
                     <% end %>
+
+                    <% if policy(@media_asset).destroy? && @media_asset.post.nil? && !@media_asset.deleted? && !@media_asset.expunged? %>
+                      <% menu.item do %>
+                        <hr class="border">
+                      <% end %>
+
+                      <% menu.item do %>
+                        <%= link_to media_asset_path(@media_asset), class: "text-danger", remote: true, xhr: true, method: :delete, "data-confirm": "This will permanently delete this file. Are you sure?" do %>
+                          <%= delete_icon %> Delete file
+                        <% end %>
+                      <% end %>
+                    <% end %>
                   <% end %>
                 </span>
               </div>