Handle zipped uploads

[simonhamp]

It would be great if CSV Import could handle zipped (.gz, .zip) files.

This would allow for larger CSV files to be uploaded as they're typically much smaller when compressed.

It would also open the door to uploading batches of files.

For the first pass, where the only file in the compressed file is the target upload:

1. Detecting that the uploaded file is of a supported compressed type
2. Decompressing the file
3. Reading the uncompressed file

Later, we could handle compressed files that contain multiple target files, but first we will probably need to build multi-file handling into other parts of the process.