Document and improve how to add behavior to Gobierto with plugins

[amiedes]

It's not a priority right now, but I think it's nice to keep this in mind.

Description

In the two Railtie plugins we've made so far we're staring to have a pattern:

1. Webpack needs to be aware of the new js files:

Webpacker::Compiler.watched_paths += [
  "app/javascript/packs/*.js",
  "app/javascript/custom_fields_data_grid_plugin/*.js",
  "app/javascript/custom_fields_data_grid_plugin/**/*.js"
]

2. We need to load Gobierto models

conf.autoload_paths += Dir[Pathname.new(base_path).join('app', 'models')]
conf.eager_load_paths += Dir[Pathname.new(base_path).join("app", "models")]

3. We have a generic way of inserting custom modules into Gobierto classes, so each plugin can add new behavior. Right now it's only used for HR/indicators/budgets plugins, but this would have been useful when we had to adapt Gobierto classes for gencat:

def attach_module(config, origin, module_class)
  if config.plugins_attached_modules[origin]
    config.plugins_attached_modules[origin].append(module_class)
  else
    config.plugins_attached_modules[origin] = [module_class]
  end
end

# ... Railtie declaration ...

attach_module(conf, "::GobiertoPlans::CategoryTermDecorator", ::GobiertoPlans::CategoryTermDecoratorBudgetsAttachment)

Proposed solution

Create a generic Gobierto::Plugin < Rails::Railtie that declares the attach_module method, and loads gobierto models and js files. Then a plugin can inherit from this class, and to modify/add new methods to a class only has to call:

attach_module(conf, "<GobiertoClass>", CustomModule)