[46] Adjust Custom Fields

[daaray]

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 removed 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.

[mlavin]

Migrations are a little awkward as noted but that can be fixed later. 🍔