Replace the existing JKAN editing UI with links to Netlify CMS

[timwis]

Once all the other tasks are complete, we'll be able to replace the editing buttons with links to edit in the Netlify CMS UI (if it's enabled for the JKAN instance), and remove all the built-in editing code from JKAN.

As @BryanQuigley suggested, we can even make it clear to viewers that we welcome them proposing changes via pull request through the UI, which Netlify CMS supports via its open authoring feature (another thing we may want to let JKAN admins enable/disable).