Test folding the OHW19-21 built files into `source`

@abkfenris and I have discussed the current website infrastructure; see #237 and #249 for past, still-current discussions of the core issues. As a first step to simplifying the current setup and exploring a potential move to ReadTheDocs (#249), we're going to test folding into the current (starting with OHW22) site files the Jekyll-based built files from OHW19 to OHW21 (note: OHW18 files are in a separate repo, https://github.com/oceanhackweek/ohw2018). That'll be an incremental step towards deprecating the release/version-based hosting of github pages built files from OHW19-21.

After copying the OHW19-21 files, I'll test the Sphinx rendering locally and create a PR to test it on the Netlify preview.

Note: we use the master branch as the github pages branch.