[events/recurrence] Cap the recurrence field to column max for now.

Summary:
Cap recurrence field so we don't throw an exception when it's too
long. This is hacky-solution, however since we aren't supporting recurrence or
syncback for now, this solution is a sufficient stop-gap approach to avoid the
exception.

Fixes T385

Test Plan: regression + code review

Reviewers: emfree

Reviewed By: emfree

Maniphest Tasks: T385

Differential Revision: https://review.inboxapp.com/D425