Google API integration: Drive, Calendar, picker.

This PR creates a new plugin tree, ggrc_gdrive_integration, with its own migrations, configurations, and assets.

GDrive integration extends Programs and the Audit objects to link to GDrive folders, and Documents to link to GDrive files, through ObjectFolder and ObjectFile join classes.

With GDrive enabled, pages have access to Drive objects through models in the CMS hierarchy (CMS.Models.GDriveFile, CMS.Models.GDriveFilePermission, etc.).  These are pulled directly from GAPI into the client.

Programs and PBC items have their info and tree views extended to show folder information.

There's a new Mustache helper, render_hooks, which adds in content by arbitrary string keys representing hook names.  Hooks are registered by GGRC.register_hook()

render_hooks is only currently called in places where it's needed, but can be extended for further use later.

PBC Response evidence is now created through the Google Picker widget.

Folders on GDrive are created when new Programs/Audits/Requests are created.

Permissions are managed on objects linked to GDrive folders, such that when owner or assignee changes, the new owner or assignee is given "writer" access to the associated folder.