[events] Make event summaries longer, truncate if longer than new length.

Summary:
We previously hit an error when an event subject was too long. Make this column longer and truncate event summaries if they are longer than that.

Fixes T296

Test Plan: Review code.

Reviewers: emfree

Reviewed By: emfree

Maniphest Tasks: T296

Differential Revision: https://review.inboxapp.com/D325