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/tests/test-mentions.md

@@ -0,0 +1,13 @@
+# Mentions Testing 
+
+To test the following mention functional: 
+
+1. Add a user 'alice' to the system
+2. Paste the below text to test if mentions is properly highlighting
+
+
+```
+To run this test, if a user named @alice doesn't yet exist, create one. 
+
+I saw @alice--and I said "Hi @alice!" then "What's up @alice?" and then @alice, was totally @alice; she just "@alice"'d me and walked on by. That's @alice...
+```