hugolib: Add disableAliases

Note that even with this setting enabled, the aliases themselves are preserved on the pages.

The big motivation for this change is to be able to use the alias definitions to generate `.htaccess` or Netlify's `_redirect` files with server-side redirects.

Fixes #3613

hugolib/config.go

@@ -131,4 +131,5 @@ func loadDefaultSettingsFor(v *viper.Viper) {
 	v.SetDefault("enableMissingTranslationPlaceholders", false)
 	v.SetDefault("enableGitInfo", false)
 	v.SetDefault("ignoreFiles", make([]string, 0))
+	v.SetDefault("disableAliases", false)
 }