This takes a list of usernames and exports their portfolios in LEAP2A format.

The end result is a big zipfile containing:
- CSV listing of each username with its associated LEAP2A zipfile
- a users/ directory containing all of the zipfiles

If an institution is selected and the username text box is left empty,
then all users for that institution will be exported.

Otherwise if there are any usernames specified in the textbox, only these
users will be exported.

Signed-off-by: Francois Marier <francois@catalyst.net.nz>