introduce plugin hook for global assets

OpenProject plugins may now register assets to be included globally
on every OpenProject page.

This should be usefull for Tracking plugins, AB-Testing, and similar
things.

If you use the openproject-plugins gem, do the following to register
your asset:

<pre>
module OpenProject::MyFancyPlugin
  class Engine < ::Rails::Engine
    engine_name :openproject_my_fancy_plugin

    include OpenProject::Plugins::ActsAsOpEngine

    register 'openproject-MyFancyPlugin',
             :author_url => 'http://openproject.org',
             :requires_openproject => '>= 3.0.0',
             :global_assets => {js:  'fancy.js',
                                         css: ['cool.css', 'another.css']}

    initializer 'my_fancy_plugin.precompile_assets' do |app|
      app.config.assets.precompile +=
['fancy.js','cool.css','another.css']
    end
  end
end
</code>