Corpus form: field validation not run

When you create a corpus using the corpus form, the `validators` for the `Field` model are never run. (This does not happen automatically when you use `save()` on a model instance, it would need to be explicitly called.)

`Field` contains two validators. One of these is a set of permitted characters; this is always met if you use the form, which will generate field names as slugs. The other is to check for overlap with forbidden field names; this can happen, and would lead to serious bugs when searching the corpus.

Solution:
1. Field validators should be run when creating a corpus through the API.
2. Ideally, this specific issue of forbidden names is avoided altogether. I'll make a separate issue for that.