R1:8b8a3142b310

Support export of data in files larger than 8MB

Summary:
Depends on D18952. Ref T13049. For files larger than 8MB, we need to engage the chunk storage engine. `PhabricatorFile::newFromFileData()` always writes a single chunk, and can't handle files larger than the mandatory chunk threshold (8MB).

Use `IteratorUploadSource`, which can, and "stream" the data into it. This should raise the limit from 8MB to 2GB (maximum size of a string in PHP).

If we need to go above 2GB we could stream CSV and text…