Add caching to images

[ratik21]

What does this do?

• Puts the downloaded images to a cache (browser cache API + indexedDB), for faster retrieval later.
• When the user reloads, it will fetch it from cache first, and if found, it will be returned. This should help in better reload times.
• Also added a cleanUp function to run a clearance script every 2 hours, this will refresh the cache i.e get rid of older entries which haven't been accessed since the last "2 days".