Refactored image handling

[martijnvdwal]

• Removed image, imageURL, imageData and imageSendRequest from the Message object.
• Created ParleyImageDataSource interface and implemented it in ParleyEncryptedImageDataSource.
  • Also created a Unit Test which tests ParleyEncryptedImageDataSource.
• Created ImageRepository to store and fetch images, including caching fetched images.
• Removed fetching UIImage objects directly from the ParleyNetworkSession (this now happens through the ImageRepository)
• Updated minimum iOS version from iOS 12 to iOS 13.