Open Authoring bugfixes and pagination improvements (#2523)

* Fix handling of displayURLs which are strings

* Add fromFetchArguments to unsentRequest

* Add parseLinkHeader to backendUtil

* Handle paginated endpoints in GitHub API

* Rename fork workflow to Open Authoring across the whole repo

* Fixes for bugs in GitHub API introduced by Open Authoring changes

* Fix getDeployPreview

* Fix incorrect auth header formatting GitHub implementation

cf. https://github.com/netlify/netlify-cms/pull/2456#discussion_r309633387

* Remove unused and broken method from GitHub API

cf. https://github.com/netlify/netlify-cms/pull/2456#discussion_r308687145

* Fix editorialWorkflowGit method in GitHub API

* Request published entry content from origin repo

* Better error when deleting a published post in Open Authoring

* Rename to Open Authoring in fork request message

Also adds a note to the fork request message that an existing fork of
the same repo will be used automatically.

* fix linting

website/content/docs/open-authoring.md

@@ -17,17 +17,17 @@ At the same time, any contributors who _do_ have write access to the repository
 
 - Your repo on GitHub must be public.
 
-## Enabling the Open Authoring
+## Enabling Open Authoring
 
 1. [Enable the editorial workflow](/docs/configuration-options/#publish-mode) by setting `publish_mode` to `editorial_workflow` in your `config.yml`.
 
-2. Set `fork_workflow` to `true` in the `backend` section of your `config.yml`, as follows:
+2. Set `open_authoring` to `true` in the `backend` section of your `config.yml`, as follows:
 
     ```yaml
     backend:
       name: github
       repo: owner-name/repo-name # Path to your GitHub repository
-      fork_workflow: true
+      open_authoring: true
     ```
 
 ## Usage