Admin/User views cannot handle both xattr and non-xattr simultaneously

Expected behavior

Admin or user created views are able to handle both mobile docs with extended attributes or docs with mobile metadata in the body.

Actual behavior

Admin or user created views can only handle the document types (xattr vs body) specified in the Sync Gateway config at the time that the view was created.
This is undesirable as it can cause situations where the result set of the views will be incomplete once the import process has begun and there's no way to get a consistent dataset.
Even ignoring this, it means that you would have to re-deploy your view after enabling shared_bucket_access and import.

Steps to reproduce

1. Turn on Sync Gateway with shared_bucket_access and import disabled
2. Add some documents to Sync Gateway
3. Create an admin view using Sync Gateway
4. Enable shared_bucket_access and import
5. Begin to query your view and check the result set, notice how this is incomplete

Migrated from https://github.com/couchbase/sync_gateway/issues/3913