Memory leak and app crash fixes

[nivanchikov]

This PR fixes a couple of issues I faced during the project onboarding

• the app crashed when the photo library permissions were accepted since the PHPhotoLibrary callback is issued on a background thread and the app tried to run UI-related tasks from there
• there was quite a large memory leak when the SingleImageViewController was being presented and dismissed. The leak was caused by the controller having a strong reference to a toolbar instance, and the toolbar had a strong delegate reference to the view controller thus causing a retain cycle. See the memory comparison before and after the fix below