Allow to hide / deregister plugins from configuration

[yakky]

Sometimes, even if a plugin is available in the codebase, you don't want it to be available in any place.
Currently CMS_PLACEHOLDER_CONF only allow to whitelist plugins in specific placeholders.
Blacklisting plugins should be available too (either globally or per-placeholder)

See also http://stackoverflow.com/questions/35044928/django-cms-placeholder-plugin-avaliable-only-to-a-specific-placeholder

[yakky]

Linked to #4979

[yakky]

Fixed by #5412
To implement blacklist, add the plugin to be blacklisted in CMS_PLACEHOLDER_CONF[None]['excluded_plugins'] list

[marcosguedes]

Started from #5000, now we here :) Updated stackoverflow answer above

[yakky]

@marcosguedes awesome!