Allow uploading testcases archive in AWS

This feature allows admins to upload all tests for a task at once, in zip-archive. Though when the contest is ready everybody uses loaders, sometimes during preparation and testing we need to upload new bunch of tests, and not all problem authors are able to do it with console script (and not all admins want to do this for them).

Zip-archive should contain only files named in some format that can be specified on upload page. Files with names not matching the format will be ignored. If a test with given codename already exists in the system, it will be skipped, unless "overwrite" option is specified.