feat(backend-github): GitHub GraphQL API support (#2456)

* add GitHub GraphQL api initial support

* support mutiple backends for e2e tests - initial commit

* add github backend e2e test (currently skipped), fix bugs per tests

* refactor e2e tests, add fork workflow tests, support fork workflow in GraphQL api

* remove log message that might contain authentication token

* return empty error when commit is not found when using GraphQL (align with base class)

* disable github backend tests

* fix bugs introduced after rebase of GraphQL and OpenAuthoring features

* test: update tests per openAuthoring changes, split tests into multiple files

* fix: pass in headers for pagination requests, avoid async iterator as it requires a polyfill on old browsers

* test(e2e): disable github backend tests

dev-test/backends/github/config.yml

@@ -0,0 +1,65 @@
+backend:
+  name: github
+  branch: master
+  repo: owner/repo
+
+publish_mode: editorial_workflow
+media_folder: static/media
+public_folder: /media
+collections:
+  - name: posts
+    label: Posts
+    label_singular: 'Post'
+    folder: content/posts
+    create: true
+    slug: '{{year}}-{{month}}-{{day}}-{{slug}}'
+    preview_path: 'posts/{{slug}}/index.html'
+    fields:
+      - label: Template
+        name: template
+        widget: hidden
+        default: post
+      - label: Title
+        name: title
+        widget: string
+      - label: Draft
+        name: draft
+        widget: boolean
+        default: true
+      - label: Publish Date
+        name: date
+        widget: datetime
+      - label: Description
+        name: description
+        widget: text
+      - label: Category
+        name: category
+        widget: string
+      - label: Body
+        name: body
+        widget: markdown
+      - label: Tags
+        name: tags
+        widget: list
+  - name: pages
+    label: Pages
+    label_singular: 'Page'
+    folder: content/pages
+    create: true
+    slug: '{{slug}}'
+    preview_path: 'pages/{{slug}}/index.html'
+    fields:
+      - label: Template
+        name: template
+        widget: hidden
+        default: page
+      - label: Title
+        name: title
+        widget: string
+      - label: Draft
+        name: draft
+        widget: boolean
+        default: true
+      - label: Body
+        name: body
+        widget: markdown