MM-27909: Add manage_shared_channels permission (#15601)

* MM-27909: Add manage_shared_channels permission

We add a new permission to manage shared channels.

It's a channel scoped permission and only the system admin
has that by default.

https://mattermost.atlassian.net/browse/MM-27909

* change to system scoped

* Trigger CI

* Trigger CI

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

model/migration.go

@@ -21,4 +21,5 @@ const (
 	MIGRATION_KEY_ADD_SYSTEM_CONSOLE_PERMISSIONS              = "add_system_console_permissions"
 	MIGRATION_KEY_SIDEBAR_CATEGORIES_PHASE_2                  = "migration_sidebar_categories_phase_2"
 	MIGRATION_KEY_ADD_CONVERT_CHANNEL_PERMISSIONS             = "add_convert_channel_permissions"
+	MIGRATION_KEY_ADD_MANAGE_SHARED_CHANNEL_PERMISSIONS       = "manage_shared_channel_permissions"
 )