change download-pdf-button => file_download

ID must be changed so to be consistent with google analytics tracking.

Issue:
- scientist-softserv/palni-palci#1020

app/assets/stylesheets/hyku.scss

@@ -428,7 +428,7 @@ div#collapse-citations {
 }
 
 @media (min-width: 576px) {
-  .citations-button .download-pdf-button {
+  .citations-button .file_download {
     max-width: 300px;
   }
 }
@@ -552,7 +552,7 @@ span.constraint-value p, .facet-values p {
   }
 }
 
-#download-pdf-button {
+#file_download {
   margin: 10px 0 10px 0;
 }
 

app/views/hyrax/base/_download_pdf.html.erb

@@ -2,10 +2,10 @@
   <div class="download-pdf-controls">
     <% if @presenter.representative_presenter.present? && presenter.file_set_presenters.any?(&:pdf?) %>
       <%= button_tag type: 'button',
-                     id: "download-pdf-button",
+                     id: "file_download",
                      data: { label: @presenter.representative_presenter.id,
                              path: hyrax.download_path(presenter.representative_presenter) },
-                     class: "btn btn-success btn-block download-pdf-button center-block",
+                     class: "btn btn-success btn-block file_download center-block",
                      onclick: "window.open(this.dataset.path, '_blank');" do %>
         Download PDF
       <% end %>