Docx tests: rewrite mediabag tests.

This will allow us to test the whole mediabag (making sure, for example,
that images are added with the correct keys) instead of just individual
extracted images.

At the moment it uses an ad-hoc Aeson-based de/serialization of the
MediaBag to JSON (bytestrings as b64 strings). If the type or its
show/read formats change, we'll switch to that.