Move enterprise features under Channels (#19010)

* Move enterprise features under Channels

We move the EE features which are Channels related.

While here, we also move some code under *Server.Start()
from NewServer.

```release-note
NONE
```

* move saml and ldap back to server

```release-note
NONE
```

* fix test

```release-note
NONE
```

* try again

```release-note
NONE
```

app/channels.go

@@ -8,6 +8,7 @@ import (
 	"sync/atomic"
 
 	"github.com/mattermost/mattermost-server/v6/app/request"
+	"github.com/mattermost/mattermost-server/v6/einterfaces"
 	"github.com/mattermost/mattermost-server/v6/model"
 	"github.com/mattermost/mattermost-server/v6/plugin"
 	"github.com/mattermost/mattermost-server/v6/services/imageproxy"
@@ -18,6 +19,8 @@ import (
 type Channels struct {
 	srv *Server
 
+	postActionCookieSecret []byte
+
 	pluginCommandsLock     sync.RWMutex
 	pluginCommands         []*PluginCommand
 	pluginsLock            sync.RWMutex
@@ -37,6 +40,11 @@ type Channels struct {
 	cachedDBMSVersion string
 	// previously fetched notices
 	cachedNotices model.ProductNotices
+
+	AccountMigration einterfaces.AccountMigrationInterface
+	Compliance       einterfaces.ComplianceInterface
+	DataRetention    einterfaces.DataRetentionInterface
+	MessageExport    einterfaces.MessageExportInterface
 }
 
 func init() {
@@ -50,6 +58,23 @@ func NewChannels(s *Server) (*Channels, error) {
 		srv:        s,
 		imageProxy: imageproxy.MakeImageProxy(s, s.httpService, s.Log),
 	}
+	// We are passing a partially filled Channels struct so that the enterprise
+	// methods can have access to app methods.
+	// Otherwise, passing server would mean it has to call s.Channels(),
+	// which would be nil at this point.
+	if complianceInterface != nil {
+		ch.Compliance = complianceInterface(New(ServerConnector(ch)))
+	}
+	if messageExportInterface != nil {
+		ch.MessageExport = messageExportInterface(New(ServerConnector(ch)))
+	}
+	if dataRetentionInterface != nil {
+		ch.DataRetention = dataRetentionInterface(New(ServerConnector(ch)))
+	}
+	if accountMigrationInterface != nil {
+		ch.AccountMigration = accountMigrationInterface(New(ServerConnector(ch)))
+	}
+
 	// Setup routes.
 	pluginsRoute := ch.srv.Router.PathPrefix("/plugins/{plugin_id:[A-Za-z0-9\\_\\-\\.]+}").Subrouter()
 	pluginsRoute.HandleFunc("", ch.ServePluginRequest)
@@ -75,6 +100,10 @@ func (ch *Channels) Start() error {
 	if err := ch.ensureAsymmetricSigningKey(); err != nil {
 		return errors.Wrapf(err, "unable to ensure asymmetric signing key")
 	}
+
+	if err := ch.ensurePostActionCookieSecret(); err != nil {
+		return errors.Wrapf(err, "unable to ensure PostAction cookie secret")
+	}
 	return nil
 }