Move processing of make_validation_result to celery (bug 957158)

https://bugzilla.mozilla.org/show_bug.cgi?id=957158

This moves the escaping of the validation code to the celery task by storing it in the `escaped_validation` column.

After the changes for an addon that fails on production:
```
It took 0.0557799339294 seconds to run upload_validation_context
It took 0.102946043015 seconds to run json_upload_detail
```

Before the changes, same addon:
```
It took 15.9265568256 seconds to run upload_validation_context
It took 15.8241832256 seconds to run json_upload_detail
```
I'm nitpicking here, but I believe doing something like the following would be a bit easier to understand:
```python
upload = FileUpload.objects.get(id=upload.pk)  # Reloading from the database.
```

This way it's clear that we want to check that the field has been saved to the database.

Another way to go would be to skip the database altogether by mocking the ```FileUpload.save``` method, and checking that it's been called by a call to ```escaped_validation()```.