Add viewer URL to StartupJS

Added viewerURL to StartupJS in order to load the data at the start of the page load.

Changelog: performance

app/views/projects/blob/_viewer.html.haml

@@ -5,6 +5,7 @@
 - external_embed = local_assigns.fetch(:external_embed, false)
 
 - viewer_url = local_assigns.fetch(:viewer_url) { url_for(safe_params.merge(viewer: viewer.type, format: :json)) } if load_async
+- add_page_startup_api_call viewer_url
 .blob-viewer{ data: { type: viewer.type, rich_type: rich_type, url: viewer_url, path: viewer.blob.path }, class: ('hidden' if hidden) }
   - if render_error
     = render 'projects/blob/render_error', viewer: viewer