ZIM files moved from cache instead of copied

If not using --keep-downloads, then ZIM files are moved instead of copied then removed
- reduces space usage
- faster if on same disk
7 jobs from move-zims-from-package-cache in 2 minutes and 44 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #155730
Code Coverage

00:02:35

passed #155731
Code Quality

00:00:42

passed #155732
Data Migration

00:02:44

passed #155733
Missing Migrations

00:00:33

passed #155729
Tests: C locale

00:02:26

passed #155727
Tests: Jessie/Python-3.4

00:02:31

passed #155728
Tests: Stretch/Python-3.5

00:02:08