Simplify MultipleContentTypeField boilerplate

[glennmatthews]

Proposed Changes

Followup to #92 - Provide a custom __init__ method for the MultipleContentTypeField class, so that we can more simply instantiate it, along the lines of:

source_type = MultipleContentTypeField(feature="relationships", required=False, label="Source Type")

Possibly something similar should also be implemented for CSVMultipleContentTypeField?

Justification

Currently there's a lot of boilerplate in instantiating a MultipleContentTypeField, particularly in constructing and ordering the queryset correctly.

    source_type = MultipleContentTypeField(
        queryset=ContentType.objects.filter(FeatureQuery("relationships").get_query()).order_by("app_label", "model"),
        required=False,
        label="Source Type",
        widget=StaticSelect2Multiple(),
    )

    destination_type = MultipleContentTypeField(
        queryset=ContentType.objects.filter(FeatureQuery("relationships").get_query()).order_by("app_label", "model"),
        required=False,
        label="Destination Type",
        widget=StaticSelect2Multiple(),
    )

It would be nice if we could instantiate it with a simple feature="relationships" and have the appropriate queryset get automatically constructed and ordered. It also probably makes sense for this field to default to using a StaticSelect2Multiple widget instead of requiring that to be manually specified each time.

[jedelman8]

@glennmatthews can you take this for 1.0.0 ?