Add and apply Rails/HttpStatus rubocop rule

We were already using it in most places. Since rubocop-rails 2.11.0,
this rule also detects offenses when using the `head` method, which we
were using with a plain `404`.

.rubocop.yml

@@ -245,6 +245,9 @@ Rails/HasManyOrHasOneDependent:
   Enabled: true
   Severity: refactor
 
+Rails/HttpStatus:
+  Enabled: true
+
 Rails/InverseOf:
   Enabled: true
   Exclude:

app/controllers/direct_uploads_controller.rb

@@ -21,7 +21,7 @@ class DirectUploadsController < ApplicationController
                      attachment_url: @direct_upload.relation.attachment.url }
     else
       render json: { errors: @direct_upload.errors[:attachment].join(", ") },
-             status: 422
+             status: :unprocessable_entity
     end
   end
 

app/controllers/pages_controller.rb

@@ -14,6 +14,6 @@ class PagesController < ApplicationController
       render action: params[:id]
     end
   rescue ActionView::MissingTemplate
-    head 404, content_type: "text/html"
+    head :not_found, content_type: "text/html"
   end
 end