MM-15021: Adding new builtin system schema for Guests (#10601)

* MM-15021: Adding new builtin system schema for Guests

* Fixing tests

* Setting properly the permissions

* Adding guests to sampledata

* Restrict more roles updates in the app layer for guests

* Adding comment to explain that permissions migration must go at the end

* Setting the default guest role for custom scheme during migration

* Fixing import and export

* Creating scheme guest roles on migration

* Fixing tests

* Fixing tests

* Fixing tests

app/scheme.go

@@ -61,8 +61,10 @@ func (a *App) CreateScheme(scheme *model.Scheme) (*model.Scheme, *model.AppError
 	// Clear any user-provided values for trusted properties.
 	scheme.DefaultTeamAdminRole = ""
 	scheme.DefaultTeamUserRole = ""
+	scheme.DefaultTeamGuestRole = ""
 	scheme.DefaultChannelAdminRole = ""
 	scheme.DefaultChannelUserRole = ""
+	scheme.DefaultChannelGuestRole = ""
 	scheme.CreateAt = 0
 	scheme.UpdateAt = 0
 	scheme.DeleteAt = 0