Improve media library performance

This MR improves the layout performance and scrolling speed of the media library as especially noticeable on slower devices such as iPhone 4S or iPad 4th gen.

It does so by:

• re-adding the thumbnail cache known from the previous media library UI
• removing 3 stack views from movie collection view cell
• merging 2 labels in movie collection view cell by using a format string
• removing 1 stack view from media collection view cell and merging 2 labels by using a format string