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/config.mk

@@ -0,0 +1,6 @@
+# Do not modify this file, if you want to configure your own environment copy
+# this file in config.override.mk and modify that file, or defining environment
+# variables using the same names found here.
+
+# Run the client in the background
+RUN_CLIENT_IN_BACKGROUND ?= true