A better move post history on sidebar

After some discussion with @treyhunner about how it seemed awkward to use a template tag to wrap context usage in a template, I made this branch to compete with @paulcollinsiii's tag solution for step 7 in the workshop in https://github.com/pythonsd/test-driven-django-development/pull/10.

The 'Make it work' section is still his version of the testing and needs to be updated to this implementation.