Address Python 3 string encoding issues in filethumbs.py
Address Python 3 string encoding issues in filethumbs.py

A mixture of issues arising when trying to write to files and
rendering static content.

edx/xblock-sdk,edx/xblock-sdk,stvstnfrd/xblock-sdk,stvstnfrd/xblock-sdk,stvstnfrd/xblock-sdk,edx/xblock-sdk