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/logo.mjml

@@ -0,0 +1,5 @@
+<mj-section padding="0px 0px 40px 0px">
+  <mj-column>
+    <mj-image mj-class="logo" src="{{.Props.SiteURL}}/static/images/logo_email_dark.png" />
+  </mj-column>
+</mj-section>