[Media Display] Cache image and preview data

[mplorentz]

Now that we are downloading media and OpenGraph metadata with custom code we need to manage caches ourselves. We don't want to re-download the same media over and over if the user scrolls past it multiple times or in different views of the app.

We should cache data in memory and/or on disk and invalid cached objects when the cache grows to big or when the app receives an low memory warning.