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 don't know how relevant it is in that case but that might be a good candidate for some kind of `JSONField` (there is [an app for that](https://pypi.python.org/pypi/django-json-field))