Allow Calendar imports to be configured with hourly or daily auto-updates

Summary:
Ref T10747. For URI-based (and, in the future, Google-based) imports, we can automatically refresh them periodically.

(In the general case there's no way to get a push notification for an ICS file, so we just have to do this every-so-often.)

Test Plan:

• Set an ICS file to update hourly.
• Used bin/trigger fire --id ... to fire it artificially.
• Saw Calendar update.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16752