check size of actual zip contents against quota

because the zip central directory can lie

test plan:
 1. Create a huge file (say a gigabyte) where every byte is the
    same.
 2. Zip it. The repeating data will be compressed. The ZIP will
    be tiny but will uncompress to a huge file.
 3. Edit the zip file binary. Open it with a hex editor and
    change the file size in the directory. Make the file
    appear to be small enough to fit into the course quota.
    (The zip file format is documented at
    http://www.pkware.com/documents/casestudies/APPNOTE.TXT
    or ask the committer for help)
 4. Try to import the zip file into a course (migrations/
    import zip file into folder)
 5. You should receive a quota error.

fixes CNVS-10722

Change-Id: Ib1bd1c432ef900f0c6c61ebe6eab2881f8515104
Reviewed-on: https://gerrit.instructure.com/29704
Reviewed-by: Mark Severson <markse@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>