Fix epub generation

- Need to run generated epub through Calibre's `ebook-convert` tool to create a
  cross-platform version of the epub. Added a dependency in the `Makefile` to
  ensure that we have the utility before attempting to build the epub, and the
  `epub` target now passes the Sphinx-generated through `ebook-convert`.
Merge branch 'hotfix/epub-generation' into develop

Fix epub generation