SPARK-2380: make images downloading more secure

[stokito]

1. Make HEAD request to check file size before downloading.
2. Ignore incomplete downloads: connection timeouts etc.
3. Move the links rendering logic to a separate package preview so that in future it may be extended to show thumbnails, audio messages etc.
4. Move download logic close to FileTransfers because eventually they should be merged (e.g. store downloaded files locally in a cache, support links to OMEMO aegis:// files, etc)