Fix deprecation message with assets

Assets in the public folder should have an extra
param if they are not using the assets pipeline.

app/controllers/projects/mattermosts_controller.rb

@@ -30,7 +30,7 @@ class Projects::MattermostsController < Projects::ApplicationController
   def configure_params
     params.require(:mattermost).permit(:trigger, :team_id).merge(
       url: service_trigger_url(@service),
-      icon_url: asset_url('slash-command-logo.png'))
+      icon_url: asset_url('slash-command-logo.png', skip_pipeline: true))
   end
 
   def teams

app/views/projects/services/slack_slash_commands/_help.html.haml

@@ -57,7 +57,7 @@
         .form-group
           = label_tag nil, _('Customize icon'), class: 'col-12 col-form-label label-bold'
           .col-12
-            = image_tag(asset_url('slash-command-logo.png'), width: 36, height: 36, class: 'mr-3')
+            = image_tag(asset_url('slash-command-logo.png', skip_pipeline: true), width: 36, height: 36, class: 'mr-3')
             = link_to(_('Download image'), asset_url('gitlab_logo.png'), class: 'btn btn-sm', target: '_blank', rel: 'noopener noreferrer')
 
         .form-group