Issue #2343 : Option to prepopulate a placeholder

Hi,

This is some code, test and doc for the issue #2343 to add default contents to a placeholder. To use it : 

```python
CMS_PLACEHOLDER_CONF = {
    'content': {
        'plugins': ['TextPlugin',],
        'default_plugins':[
            {
                'plugin_type':'TextPlugin', 
                'value':{'body':'<p>Lorem ipsum dolor sit amet...</p>'},
            },
        ]
    },
}```

I don't think the assign_plugins function is the best place to do that, so I would be happy if you could advice me (I'm a newbie with Django, Django CMS and Python ^^)