Adds a category filter for NewsBlogFeaturedArticlesPlugin and NewsBlogLatestArticlesPlugin

[corentin-cres]

Allows the configuration of a filter on categories for the plugins based on NewsBlogCMSPlugin.

This filter is either :

• A list of categories (multiple choice field) for this specific plugin ;
• Or can be dynamically determined for a given request. (Boolean field)

If the dynamic determination is specified, the list of categories will be ignored.
If no category is selected, it will not use any filter on categories ; so you won't have to go through your existing NewsBlogCMSPlugins on your website to update the configuration.

Dynamic configuration

This functionality is based on a new setting, ALDRYN_NEWSBLOG_CATEGORIES_FROM_REQUEST.
For example, if you map some categories in your page (page model has a categories multiple choice field) ; you will be able to use this mapping in your NewsBlogLatestArticlePlugin.

In this example, just set in your settings.py :

ALDRYN_NEWSBLOG_CATEGORIES_FROM_REQUEST = 'current_page.categories'

Basically, it will getattr(request, ALDRYN_NEWSBLOG_CATEGORIES_FROM_REQUEST).
So, in this example, it will filter using request.current_page.categories.

If the setting ALDRYN_NEWSBLOG_CATEGORIES_FROM_REQUEST is left empty, it will be ignored by the plugin and it will fallback using the defined list of categories.
The same goes if request.current_page.categories is set to an empty list of categories.