Move script loader after webpack manifest (#3569)
Move script loader after webpack manifest (#3569)

This should ensure that when any external scripts are loaded using
script loader, the webpack manifest is already present.
mingaldrichgan/gatsby,danielfarrell/gatsby,fabrictech/gatsby,fk/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,0x80/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,danielfarrell/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,fk/gatsby,fk/gatsby,gatsbyjs/gatsby,0x80/gatsby,danielfarrell/gatsby,0x80/gatsby,mingaldrichgan/gatsby,fabrictech/gatsby,fabrictech/gatsby,ChristopherBiscardi/gatsby