New importer

[RealOrangeOne]

Add a new importer, developed internally at Torchbox for a client project.

The benefits here are:

• Less code, which is easier to follow
• Performance improvements
• Expose possible errors, so they can be fed back to the end user if needed

The bulk of the work is in wagtail_airtable/importer.py

I also took the opportunity to replace the explicit usage of MockAirtable with unittest.mock.patch.

[KalobTaulien]

This looks like a huge improvement! I'll try and find some time soon to review and user test this, and then make a package version bump.

Thank you for this contribution!