Bug 1051806: Add /fxa-register endpoint.

@Sancus r?

This may conflict a bit, or at least could benefit from, the work
you're doing to move more ET stuff to tasks, but we can always clean
up after all the merging.
```python
class ImageBannerBaseTests(TestCase):
    def setUp(self):
        patcher = patch('affiliates.banners.models.render_to_string')
        self.addCleanup(patcher.stop)
        self.render_to_string = patcher.start()
        self.render_to_string.return_value = 'asdf'

        patcher = patch('affiliates.banners.models.translation.get_language')
        self.addCleanup(patcher.stop)
        self.get_language = patcher.start()
```