mime: change *.js mime type to application/javascript, not x-javascript

We delayed doing this for 4 years for fear that it might break something,
but it was standardized (RFC 4329) 12 years ago, and the default in Debian
and other places is correct:

   $ cat /etc/mime.types  | grep js$
   application/javascript                          js

Time for us to change too.

I doubt there will be problems, but we'll see during the Go 1.11 beta.

Fixes #7498

Change-Id: Iba0bf8a6e707a64dd63317e1c0d6dd9a18634527
Reviewed-on: https://go-review.googlesource.com/120058


Reviewed-by: Andrew Bonventre <andybons@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>