[BUGFIX] Manage PHP configuration with "unlimited" upload

[BUGFIX] Manage PHP configuration with "unlimited" upload

Setting `post_max_size` and `upload_max_filesize` PHP options to "0" is unmanaged by Jirafeau.
It tries to send 0 bytes and Javascript have some zero division errors.

This commit will use $max_upload_chunk_size_bytes option when available.
If $max_upload_chunk_size_bytes is also set to zero, chunk size is set an arbitrary value of 10M.

Non html-5 users will not have size limitation shown.

fixes #272

Signed-off-by: Jerome Jutteau <jerome@jutteau.fr>