[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/api4/user.go

@@ -61,6 +61,7 @@ func (api *API) InitUser() {
 	api.BaseRoutes.User.Handle("/mfa/generate", api.APISessionRequiredMfa(generateMfaSecret)).Methods("POST")
 
 	api.BaseRoutes.Users.Handle("/login", api.APIHandler(login)).Methods("POST")
+	api.BaseRoutes.Users.Handle("/login/desktop_token", api.RateLimitedHandler(api.APIHandler(loginWithDesktopToken), model.RateLimitSettings{PerSec: model.NewInt(2), MaxBurst: model.NewInt(1)})).Methods("POST")
 	api.BaseRoutes.Users.Handle("/login/switch", api.APIHandler(switchAccountType)).Methods("POST")
 	api.BaseRoutes.Users.Handle("/login/cws", api.APIHandlerTrustRequester(loginCWS)).Methods("POST")
 	api.BaseRoutes.Users.Handle("/logout", api.APIHandler(logout)).Methods("POST")
@@ -1966,6 +1967,30 @@ func login(c *Context, w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+func loginWithDesktopToken(c *Context, w http.ResponseWriter, r *http.Request) {
+	props := model.MapFromJSON(r.Body)
+	token := props["token"]
+	deviceId := props["device_id"]
+
+	user, err := c.App.ValidateDesktopToken(token, time.Now().Add(-model.DesktopTokenTTL).Unix())
+	if err != nil {
+		c.Err = err
+		return
+	}
+
+	err = c.App.DoLogin(c.AppContext, w, r, user, deviceId, false, false, false)
+	if err != nil {
+		c.Err = err
+		return
+	}
+
+	c.App.AttachSessionCookies(c.AppContext, w, r)
+
+	if err := json.NewEncoder(w).Encode(user); err != nil {
+		c.Logger.Warn("Error while writing response", mlog.Err(err))
+	}
+}
+
 func loginCWS(c *Context, w http.ResponseWriter, r *http.Request) {
 	campaignToURL := map[string]string{
 		"focalboard": "/boards",