DEV: Introduce DISCOURSE_ASSET_URL_SALT (#24596)

This value is included when generating static asset URLs. Updating the value will allow site operators to invalidate all asset urls to recover from configuration issues which may have been cached by CDNs/browsers.

app/models/javascript_cache.rb

@@ -22,7 +22,8 @@ class JavascriptCache < ActiveRecord::Base
   end
 
   def update_digest
-    self.digest = Digest::SHA1.hexdigest(content) if content_changed?
+    self.digest =
+      Digest::SHA1.hexdigest("#{content}|#{GlobalSetting.asset_url_salt}") if content_changed?
   end
 
   def content_cannot_be_nil