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/app/login.go

@@ -323,6 +323,12 @@ func (a *App) AttachSessionCookies(c *request.Context, w http.ResponseWriter, r
 		Secure:  secure,
 	}
 
+	if secure && utils.CheckEmbeddedCookie(r) {
+		sessionCookie.SameSite = http.SameSiteNoneMode
+		userCookie.SameSite = http.SameSiteNoneMode
+		csrfCookie.SameSite = http.SameSiteNoneMode
+	}
+
 	http.SetCookie(w, sessionCookie)
 	http.SetCookie(w, userCookie)
 	http.SetCookie(w, csrfCookie)