Merge branch 'sri' into 'master'

Add Subresource Integrity attribute to CSS and JS assets.

This prevents compromised or malicious CDNs from modifying GitLab's assets. The hash provided by Rails is compared to the hash of the asset the browser has downloaded. The browser will refuse to execute/parse the assets if the hashes don't match. SRI is currently implemented in Firefox, Chrome, and Opera.

This doesn't apply to the dynamically-generated per-page JavaScript due to [a bug in sprockets-rails](https://github.com/rails/sprockets-rails/issues/359). Unfortunately until there's a fix available we won't benefit fully from a security perspective.

It's more secure. More information is available in #18230 and on MDN:
https://developer.mozilla.org/en-US/docs/Web/Security/Subresource_Integrity

Fixes #18230

See merge request !4808

CHANGELOG

@@ -3,6 +3,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 v 8.10.0 (unreleased)
   - Wrap code blocks on Activies and Todos page. !4783 (winniehell)
   - Fix MR-auto-close text added to description. !4836
+  - Implement Subresource Integrity for CSS and JavaScript assets. This prevents malicious assets from loading in the case of a CDN compromise.
 
 v 8.9.0
   - Fix builds API response not including commit data

app/views/layouts/_head.html.haml

@@ -25,11 +25,14 @@
 
   = favicon_link_tag 'favicon.ico'
 
-  = stylesheet_link_tag "application", media: "all"
-  = stylesheet_link_tag "print",       media: "print"
+  = stylesheet_link_tag "application", media: "all", integrity: true
+  = stylesheet_link_tag "print",       media: "print", integrity: true
 
-  = javascript_include_tag "application"
+  = javascript_include_tag "application", integrity: true
 
+  -# FIXME: SRI doesn't apply to the dynamically-generated per-page
+  -# JavaScript due to a bug in sprockets-rails.
+  -# See https://github.com/rails/sprockets-rails/issues/359
   - if page_specific_javascripts
     = javascript_include_tag page_specific_javascripts, {"data-turbolinks-track" => true}