[MM-37984] Allow Desktop App to authenticate via external providers outside of the app on supported servers (#23795)

* WIP

* Add rate limiting for desktop token API

* Missing mocks

* Style fixes

* Update snapshots

* Maybe use an actual redirect link :P

* Refactoring for tests

* Add tests for server

* Fix lint issue

* Fix tests

* Fix lint

* Add front-end screen component

* Component logic

* Style changes

* Quick style fix

* Lint fixes

* Initial PR feedback

* Enable logging into the browser as well when completing the login process

* Refactor to push more logic to the other component

* Remove unnecessary helper code

* Fix i18n

---------

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

server/channels/web/saml.go

@@ -9,6 +9,7 @@ import (
 	"net/http"
 	"strconv"
 	"strings"
+	"time"
 
 	"github.com/mattermost/mattermost/server/public/model"
 	"github.com/mattermost/mattermost/server/public/shared/mlog"
@@ -59,6 +60,16 @@ func loginWithSaml(c *Context, w http.ResponseWriter, r *http.Request) {
 		relayProps["redirect_to"] = redirectURL
 	}
 
+	desktopToken := r.URL.Query().Get("desktop_token")
+	if desktopToken != "" {
+		desktopTokenErr := c.App.CreateDesktopToken(desktopToken, time.Now().Unix())
+		if desktopTokenErr != nil {
+			c.Err = err
+			return
+		}
+		relayProps["desktop_token"] = desktopToken
+	}
+
 	relayProps[model.UserAuthServiceIsMobile] = strconv.FormatBool(isMobile)
 
 	if len(relayProps) > 0 {
@@ -185,6 +196,26 @@ func completeSaml(c *Context, w http.ResponseWriter, r *http.Request) {
 
 	c.App.AttachSessionCookies(c.AppContext, w, r)
 
+	desktopToken := relayProps["desktop_token"]
+	if desktopToken != "" {
+		desktopTokenErr := c.App.AuthenticateDesktopToken(desktopToken, time.Now().Add(-model.DesktopTokenTTL).Unix(), user)
+		if desktopTokenErr != nil {
+			handleError(desktopTokenErr)
+			return
+		}
+
+		queryString := map[string]string{
+			"desktopAuthComplete": "true",
+		}
+		if val, ok := relayProps["redirect_to"]; ok {
+			queryString["redirect_to"] = val
+		}
+
+		redirectURL = utils.AppendQueryParamsToURL(c.GetSiteURLHeader()+"/login/desktop", queryString)
+		http.Redirect(w, r, redirectURL, http.StatusFound)
+		return
+	}
+
 	if hasRedirectURL {
 		if isMobile {
 			// Mobile clients with redirect url support