export: Export file images for single users.
export: Export file images for single users.

We don't have automated test coverage on this yet,
but below are the results from manual testing.

Note that we include the realm icon and logo even
though they were not created by Cordelia.

    ./manage.py export_single_user cordelia@zulip.com

    $ (cd /tmp/zulip-export-4v3mo802/ && find .)
    .
    ./emoji
    ./emoji/2
    ./emoji/2/emoji
    ./emoji/2/emoji/images
    ./emoji/2/emoji/images/3.jpg
    ./emoji/records.json
    ./messages-000001.json
    ./realm_icons
    ./realm_icons/2
    ./realm_icons/2/night_logo.original
    ./realm_icons/2/night_logo.png
    ./realm_icons/2/icon.png
    ./realm_icons/2/icon.original
    ./realm_icons/records.json
    ./avatars
    ./avatars/2
    ./avatars/2/c5125af0447f4d66ce34c1b32eac75ac27ebe0e7.original
    ./avatars/2/c5125af0447f4d66ce34c1b32eac75ac27ebe0e7.png
    ./avatars/records.json
    ./uploads
    ./uploads/2
    ./uploads/2/68
    ./uploads/2/68/xyEkC5dTIp8m42_6HJ3kBfdt
    ./uploads/2/68/xyEkC5dTIp8m42_6HJ3kBfdt/denver.jpg
    ./uploads/2/96
    ./uploads/2/96/ol5WE6RTUntvuPDSpJUrYTim
    ./uploads/2/96/ol5WE6RTUntvuPDSpJUrYTim/denver.jpg
    ./uploads/records.json
    ./user.json

andersk/zulip,zulip/zulip,zulip/zulip,andersk/zulip,rht/zulip,zulip/zulip,zulip/zulip,andersk/zulip,rht/zulip,rht/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,kou/zulip,andersk/zulip,rht/zulip,andersk/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,eeshangarg/zulip,rht/zulip,kou/zulip,kou/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip