Mono repo -> Master (#22553)

Combines the following repositories into one:

https://github.com/mattermost/mattermost-server
https://github.com/mattermost/mattermost-webapp
https://github.com/mattermost/focalboard
https://github.com/mattermost/mattermost-plugin-playbooks

server/.codecov.yml

@@ -0,0 +1,7 @@
+comment:
+  layout: "diff, flags, files"
+  require_changes: true
+  branches: "!release*"
+
+ignore:
+  - store/storetest