MM-53147 Support for embedding Mattermost in an MSTeams iframe (#23776)

* add teams to allowed frame-ancestors

* fix unit tests

* set SameSite attribute for session cookie

* further restrict ancestors

* skip landing page if in iframe

* Only set cookie SameSite=None if embedded in iframe

* don't set MMEMBED cookie on landing page (check only)

* fully parse MMEMBED cookie

* add comment

* more comments

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/api4/user.go

@@ -2241,6 +2241,10 @@ func attachDeviceId(c *Context, w http.ResponseWriter, r *http.Request) {
 		Secure:   secure,
 	}
 
+	if secure && utils.CheckEmbeddedCookie(r) {
+		sessionCookie.SameSite = http.SameSiteNoneMode
+	}
+
 	http.SetCookie(w, sessionCookie)
 
 	if err := c.App.AttachDeviceId(c.AppContext.Session().Id, deviceId, c.AppContext.Session().ExpiresAt); err != nil {