Make it easier to customize JavaScript functions

When there was a custom JavaScript file, we weren't loading the original
one, meaning that, in order to customize it, it was necessary to copy
the whole original file and then changing it.

Now we're loading both the original and the custom file, so the custom
file can simply add more functions or overwrite the ones we'd like to
customize, without copying the whole file.

Existing copies of original files will still overwrite the whole file
and won't be affected.

app/assets/javascripts/application.js

@@ -114,6 +114,7 @@
 //= require_tree ./admin
 //= require_tree ./sdg
 //= require_tree ./sdg_management
+//= require_tree ./custom
 
 var initialize_modules = function() {
   "use strict";

config/application.rb

@@ -114,7 +114,7 @@ end
 class Rails::Engine
   initializer :prepend_custom_assets_path, group: :all do |app|
     if self.class.name == "Consul::Application"
-      %w[images fonts javascripts].each do |asset|
+      %w[images fonts].each do |asset|
         app.config.assets.paths.unshift(Rails.root.join("app", "assets", asset, "custom").to_s)
       end
     end