`ox-gfm` is part of my pipeline for publishing my Denote notes to my
website. I will write about this sometime soon and publish the Denote
side code for it as well.
The default value of `denote-journal-extras-directory` derives from
the default value of `denote-directory`.
Since I change this default value for myself, I need to also change
the `denote-journal-extras-directory` (or more the require after my
`denote-directory` variable is defined).
Redefining it is fine with me.