Mark manifest.[go|js] as a generated file (#57)

manifest.[go|js] files are autogenerated so we dont need to
show diffs, cope with conflicts in merge operations or show the
diff in github.

server/manifest.go

@@ -1,3 +1,5 @@
+// This file is automatically generated. Do not modify it manually.
+
 package main
 
 var manifest = struct {