[keyserver] deconstruct sidebarCast fields

Summary:
Following feedback from https://phab.comm.dev/D13855, I'm trying to reduce character counts below 80.

Besides for shortening variable names, in this diff, I also opted to use deconstruction syntax to remove the need to directly access text from sidebarCast when defining messageText;

Test Plan: flow check

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13856