This change fixes up three issues I encountered with string types:

The logging method for missing cache keys in the large data utilities

  was mixing str and bytes.
The documentation for the return value of make_cache_key wasn't correct.
The new template loader for extension templates was returning contents

  as bytes, which caused problems when they were rendered (the

  resulting rendered string would include b'' around the content).
The WS_RE regexp used in the djblets_utils templatetags was

  incorrectly not marked as a raw string.

Ran unit tests.
Used templates from an extension and saw them render correctly.