vulcan - expose workspace vignettes via the 'get_workspace' endpoint

[dtm2451]

PR hooks up workflow vignettes for display from the workspace interface:

• Adds the content of the file to a vignette key in the get_workspace endpoint return, if the file exists in the workspace.
  • This endpoint is called only when the user first loads or refreshes the workspace page, so it's a low usage endpoint that is safe to bloat a bit!
• Tweaks the already in-place ' Workflow' button to target this content
• Adds a vignette to the tests 'snakemake-repo', and a line in one of the current get_workflow tests which ensure this vignette return is added.