[native] Add memoization to SidebarItem

Summary: Call to shortAbsoluteDate in particular takes surprisingly long (like 5ms each time and it's re-rendering like 5 times and there are 5 instances so 5*5*5ms = 125ms total when doing the "navigate to thread w/ varun scenario").

Test Plan: Continues to work/appear as before.

Reviewers: ginsu, tomek, rohan

Reviewed By: ginsu

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9099