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>

testlib/store.go

@@ -46,6 +46,7 @@ func GetMockStoreForSetupFunctions() *mocks.Store {
 	systemStore.On("GetByName", model.MIGRATION_KEY_ADD_USE_GROUP_MENTIONS_PERMISSION).Return(&model.System{Name: model.MIGRATION_KEY_ADD_USE_GROUP_MENTIONS_PERMISSION, Value: "true"}, nil)
 	systemStore.On("GetByName", model.MIGRATION_KEY_ADD_SYSTEM_CONSOLE_PERMISSIONS).Return(&model.System{Name: model.MIGRATION_KEY_ADD_SYSTEM_CONSOLE_PERMISSIONS, Value: "true"}, nil)
 	systemStore.On("GetByName", model.MIGRATION_KEY_ADD_CONVERT_CHANNEL_PERMISSIONS).Return(&model.System{Name: model.MIGRATION_KEY_ADD_CONVERT_CHANNEL_PERMISSIONS, Value: "true"}, nil)
+	systemStore.On("GetByName", model.MIGRATION_KEY_ADD_MANAGE_SHARED_CHANNEL_PERMISSIONS).Return(&model.System{Name: model.MIGRATION_KEY_ADD_MANAGE_SHARED_CHANNEL_PERMISSIONS, Value: "true"}, nil)
 	systemStore.On("Get").Return(make(model.StringMap), nil)
 	systemStore.On("Save", mock.AnythingOfType("*model.System")).Return(nil)