Add enum to import_preview/name

[jsangmeister]

As suggested in OpenSlides/openslides-autoupdate-service#781, we should constrain the values of this field with an enum to ensure only valid import names are entered. Also, I think that type would be more fitting as a field name.

[r-peschke]

The backend json-uploads and imports are each individual actions and each one uses and tests the name field-content individually. Therefore an enum doesn't help. It just let us extend the enum for each new import.
The fieldname name was choosen with the idea to differentiate the imports. I would suggest to improve the name-field content by a more specific name, i.e. "import motions for summer-event ...".
Written this, I see 2 fields, name and type. But do we need this now?

[jsangmeister]

As discusssed: We will add an enum for the field.