Queue large ICS files for background import

Summary: Ref T11801. When a file is larger than 512KB, queue it for background import instead of trying to do it in the foreground, sinc we risk hitting max_execution_time.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11801

Differential Revision: https://secure.phabricator.com/D16805