Fix typo in ossec package name
Save bulk downloads to SecureDrop-specific tmp dir

In order to serve bulk downloads with X-Send-File, we need to save them
in a SecureDrop-specific temp directory to avoid allowing Apache to
serve all of a default temp directory such as `/tmp`.

This commit also adds timestamps to the bulk download .zip filenames to
make them easier to work with.

Tested successfully on staging.
Add manage.py command to clean out SecureDrop's temp dir

This command is designed to be run as part of a regular cron job to
prevent this directory from accumulating past downloads and eventually
exhausting disk space. This command checks to see if a file is in use by
a process before deleting it, so it will not delete files that are in
the process of being downloaded.

Tested that this command cleans the tmp dir and does not interrupt
ongoing downloads on staging.
Ansible task sets up daily cron job to clean up SecureDrop temp dir
Merge branch 'develop' into fix-download-unread
Create SecureDrop TEMP_DIR for tests
Merge pull request #709 from freedomofpress/fix-download-unread

Fix download unread