Change default branch from 'master' to 'main' (#1589)

The default branch has been changed. This PR cleans up references to
`master` and fixes links that would otherwise be broken.

For more background see the following references:
* https://www.independent.co.uk/life-style/gadgets-and-tech/news/github-master-slave-slavery-whitelist-language-inclusive-a9568576.html
* https://tools.ietf.org/id/draft-knodel-terminology-00.html

.github/workflows/compatibility_tests.yml

@@ -5,7 +5,7 @@ on:
     - cron: '0 8 7 * 2'
   push:
     branches:
-      - master
+      - main
 
 env:
   JEST_ENV: prod
@@ -22,7 +22,7 @@ jobs:
       - name: Build docker image
         run: docker build .
 
-  cellxgene-master-with-python-and-anndata-versions:
+  cellxgene-main-with-python-and-anndata-versions:
     name: python versions x anndata versions
     runs-on: ubuntu-latest
     strategy:
@@ -86,8 +86,8 @@ jobs:
       - name: Tests
         run: cd cellxgene && make unit-test ${{ matrix.test-suite }}
 
-  cellxgene-master-with-anndata-master:
-    name: cellxgene master with anndata master
+  cellxgene-main-with-anndata-master:
+    name: cellxgene main with anndata master
     runs-on: ubuntu-latest
     strategy:
       matrix:

.github/workflows/deploy.yml

@@ -2,7 +2,7 @@ name: Deploy via single cell infra repo
 
 on:
   push:
-    branches: master
+    branches: main
 
 jobs:
   deploy:

.github/workflows/push_tests.yml

@@ -2,7 +2,7 @@ name: Push Tests
 
 on:
   push:
-    branches: master
+    branches: main
   pull_request:
     branches: "*"