MM-11120 Adding setting to disable email invitations and rate limiting. (#9063)

* Adding setting to disable email invitations.

* Adding a setting and rate limiting for email invite sending.

* Modifying email rate limit to 20/user/hour

* Adding EnableEmailInvitations to client side config and command.

app/app.go

@@ -17,6 +17,7 @@ import (
 
 	"github.com/gorilla/mux"
 	"github.com/pkg/errors"
+	"github.com/throttled/throttled"
 
 	"github.com/mattermost/mattermost-server/einterfaces"
 	ejobs "github.com/mattermost/mattermost-server/einterfaces/jobs"
@@ -46,7 +47,8 @@ type App struct {
 	IsPluginSandboxSupported bool
 	pluginStatuses           map[string]*model.PluginStatus
 
-	EmailBatching *EmailBatchingJob
+	EmailBatching    *EmailBatchingJob
+	EmailRateLimiter *throttled.GCRARateLimiter
 
 	Hubs                        []*Hub
 	HubsStopCheckingForDeadlock chan bool
@@ -185,6 +187,10 @@ func New(options ...Option) (outApp *App, outErr error) {
 
 	})
 
+	if err := app.SetupInviteEmailRateLimiting(); err != nil {
+		return nil, err
+	}
+
 	mlog.Info("Server is initializing...")
 
 	app.initEnterprise()