Create zip files for large works and collections

[awead]

Description

Defines a rake task that creates zips of public collections and works that exceed a given size. second rake task will delete zip files that no longer fit the criteria for the export.

The option to download works or collections in the UI will be presented if the resource is below the certain threshold size, or if the pre-made zip is available. If the resource exceeds the specified size and a zip file has not been created yet, then no zip download option will be present.

Changes

• adds a new ZipJob and ZipFile classes
• modifies UI to display zip download links only if valid zip files exist or can be created
• if an up-to-date zip file exists, the service will not recreate the file

[coveralls]

Coverage decreased (-31.2%) to 57.285% when pulling 20ae89ee8e7817ee23d725e3d7e15604de04ae20 on i#1590-large-downloads into 07f368ac22efc965784c82732a0e2e8412f04ea5 on develop.