MM-29011: Add feature flag for shared channels (#15585)

* MM-29011: Add feature flag for shared channels

https://mattermost.atlassian.net/browse/MM-29011

* gofmt

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

model/config.go

@@ -875,6 +875,7 @@ type ExperimentalSettings struct {
 	UseNewSAMLLibrary               *bool   `access:"experimental,cloud_restrictable"`
 	CloudUserLimit                  *int64  `access:"experimental,write_restrictable,cloud_restrictable"`
 	CloudBilling                    *bool   `access:"experimental,write_restrictable,cloud_restrictable"`
+	EnableSharedChannels            *bool   `access:"experimental"`
 }
 
 func (s *ExperimentalSettings) SetDefaults() {
@@ -910,6 +911,10 @@ func (s *ExperimentalSettings) SetDefaults() {
 	if s.UseNewSAMLLibrary == nil {
 		s.UseNewSAMLLibrary = NewBool(false)
 	}
+
+	if s.EnableSharedChannels == nil {
+		s.EnableSharedChannels = NewBool(false)
+	}
 }
 
 type AnalyticsSettings struct {