[web] Fix sidebar title cutoff in ChatThreadList

Summary:
Linear issue with full context here.

Seems to be a regression, not sure when it was introduced exactly. This diff repositions the ChatThreadListItemMenu .menuSidebar div to be absolutely positioned so that it doesn't interfere with the ChatThreadListSidebar. Before, the ChatThreadListSidebar was being covered by the ChatThreadListItemMenu and it probably went unnoticed since it wasn't tested with a sidebar message that was long enough to trigger the ellipsis overflow.

Before:

After:

Test Plan: Looks as expected on Chrome/Safari and the ChatThreadListItemMenu still works as expected.

Reviewers: atul, ashoat, palys-swm

Reviewed By: ashoat

Subscribers: palys-swm, Adrian

Differential Revision: https://phab.comm.dev/D4284