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

webapp/.gitignore

@@ -0,0 +1,12 @@
+.eslintcache
+junit.xml
+node_modules
+*.tsbuildinfo
+
+channels/dist
+playbooks/dist
+boards/dist
+platform/*/dist
+platform/*/lib
+
+config.override.mk