Revert "Used tempfile.NamedTemporaryFile instead of StringIO in wagtailimages"

This reverts commit 0d8377863e4374a0886e386170ead145e91952d3.
Workaround Django bug causing 'I/O operation on closed file' errors

The issue is caused by the following commit:
https://github.com/django/django/commit/30fc49a7ca0d030c7855f31ed44395903fa6abdd

This causes the file to be closed before storage.save function returns. Causing the following line to raise an 'I/O operation on closed file' error
https://github.com/django/django/blob/2bcb8bfc8de5cd57ebb64b326cd7609aa0d8c1c7/django/db/models/fields/files.py#L92

The workaround is simply to set the _size attribute on the content object before saving. This prevents Django from attempting to get the size of the file when it is closed.