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

packages/netlify-cms-lib-util/src/unsentRequest.js

@@ -13,6 +13,12 @@ const fromURL = wholeURL => {
   return Map({ url, ...(allParamsString ? { params: decodeParams(allParamsString) } : {}) });
 };
 
+const fromFetchArguments = (wholeURL, options) => {
+  return fromURL(wholeURL).merge(
+    (options ? fromJS(options) : Map()).remove('url').remove('params'),
+  );
+};
+
 const encodeParams = params =>
   params
     .entrySeq()
@@ -25,8 +31,8 @@ const toURL = req =>
 const toFetchArguments = req => [
   toURL(req),
   req
-    .delete('url')
-    .delete('params')
+    .remove('url')
+    .remove('params')
     .toJS(),
 ];
 
@@ -85,6 +91,7 @@ const withTimestamp = ensureRequestArg(req => withParams({ ts: new Date().getTim
 export default {
   toURL,
   fromURL,
+  fromFetchArguments,
   performRequest,
   withMethod,
   withDefaultMethod,