Prepare to rename main

Author: dannystaple

.github/workflows/on_pr_test.yaml

@@ -3,6 +3,7 @@ on:
   pull_request:
     branches:
       - master
+      - main
 
 jobs:
   build_site:

.github/workflows/on_push_to_master_test_and_deploy.yaml

@@ -4,6 +4,7 @@ on:
   push:
     branches:
       - master
+      - main
 
 concurrency:
   group: site_building

admin/config.yml

@@ -1,7 +1,7 @@
 backend:
     name: github
     repo: orionrobots/orionrobots.github.io
-    branch: master
+    branch: main
     base_url: https://github-oauth2.orionrobots.co.uk
 # This line should *not* be indented (for draft/review/publish workflow)
 # publish_mode: editorial_workflow