Journalist selects and downloads multiple files

Fixes issue #123

* Adds checkboxes alongside files in journalist view
* Adds a Download Checked button
* Server uses Python's built-in zipfile library to create and send back a zipfile with the documents

Issues: the zip file is created in the root directory and not deleted. It can't be deleted with crypto.secureunlink because it is not in the secure directory. Would it be better to put the zipfile into the secure directory and then delete, or put it in a /tmp folder and allow crypto.secureunlink to delete?