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/templates/partials/message_button.mjml

@@ -0,0 +1,5 @@
+<mj-column width=100%>
+    <mj-raw>{{if .MessageURL}}</mj-raw>
+    <mj-button href="{{.MessageURL}}" padding="16px 0px 0px 0px" css-class="messageButton">{{$.Props.MessageButton}}</mj-button>
+    <mj-raw>{{end}}</mj-raw>
+</mj-column>