[46] Adjust Custom Fields

Fixes #46 

Due to the way Mezzanine incorporates custom fields via the settings file, the migration for this PR requires special attention:

To run the migrations, in ```project.settings.base.EXTRA_MODEL_FIELDS```, you'll need to temporarily include the remove fields:

```
    (
        "mezzanine.pages.models.Page.closing",
        "TextField",  # 'django.db.models.' is implied if path is omitted.
        ("Closing Paragraph",),
        {"default": '', "blank": True},
    ),
     (
        "mezzanine.pages.models.Page.subtitle",
        "CharField",  # 'django.db.models.' is implied if path is omitted.
        ("Subtitle",),
        {"max_length": 128, "default": '', "blank": True},
    )
```

Then execute:

```python manage.py migrate sandstone```

Once that is complete, remove the temp fields from ```project.settings.base.EXTRA_MODEL_FIELDS```.

We likely should collapse the migrations at some point after these are run for fxoss, as the migration instructions for a new site or not straightforward either.