Support old devices

[serejahh]

Avoid freezes on old devices and in table view

UIVisualEffectView works better, but there's a limitation on iOS 10: https://forums.developer.apple.com/thread/50854

We have the following approaches solving the problem:

1. Use maskView instead of maskLayer
2. Perform blur in background (we need to ignore setting an image to ForceBlurImageView to prevent displaying unblurred image)

[serejahh]

Need to try http://stackoverflow.com/questions/39595047/uivisualeffectview-with-mask-layer as @asimonv advised in #6