Remove dependency of go-bindata (#818)

* Use new embed functionality for serving UI assets

* Use new embed functionality for serving resources. Remove dependency on go-bindata

* Remove Go 1.15

server/subsonic/media_retrieval.go

@@ -46,7 +46,7 @@ func (c *MediaRetrievalController) GetAvatar(w http.ResponseWriter, r *http.Requ
 }
 
 func (c *MediaRetrievalController) getPlaceHolderAvatar(w http.ResponseWriter, r *http.Request) (*responses.Subsonic, error) {
-	f, err := resources.AssetFile().Open(consts.PlaceholderAvatar)
+	f, err := resources.Assets().Open(consts.PlaceholderAvatar)
 	if err != nil {
 		log.Error(r, "Image not found", err)
 		return nil, newError(responses.ErrorDataNotFound, "Avatar image not found")