Widgets - Add support to get widgets partial HTML

[abujeda]

To create a more dynamic UX with widgets, it would be great if we could request widget HTML, without the Rails layout furniture, from another widget with AJAX.

This will allow to load the homepage faster with a widget skeleton HTML and request any other data/HTML through AJAX.

# Possible route
get '/widgets/*widget_path' => 'widgets#show'

We needed this feature when developing our metrics widget. There are cases due to the number of jobs a user has completed that the request to Slurm to get the metrics data take a long time. This is delaying the rendering of the homepage for quite a while.

Our solution for now is to move the widget into a custom page so that the homepage is not affected.