prepare for release of 2.1.7
Fix two issues:
1) building the cloned array of sources in getSource() was failing because we were setting the keys to [i] which no longer always starts with 0. Instead, push the values into the array. This was breaking JWPlayer when the RTMP source was skipped.
2) Always load the full playlist instead of just the array of sources. We'd lose the .vtt tracks and images without this.
Kill no longer necessary PACKAGE_ID

:skull: death to old cruft.
Add support for valid human readable formats from SiteAttachment.
New base abstract attachment upload class.

This can be used across all our attachment admins.
Add a site level abstract attachment downloader.

This can be reused for all of our sites that use SiteAttachment.
Bump swat dependency as well.

This is required for SwatFileEntry features that were recently added and are being used in the new Attachment upload base class.
Add license
Add a getResolvedObjectClass() and use it.
Mark xml strings as translatable
Set the form's upload_status_server in build and add a method to get it.

This will allow for easier subclassing.