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_attachment_styles.mjml

@@ -0,0 +1,5 @@
+<!-- this empty mj attribute tag is required by MJML to compile successfully -->
+<mj-attributes/>
+
+<mj-include type="css" path="message_attachment.css"/>
+<mj-include type="css" css-inline="inline" path="message_attachment.css"/>