[web] Display button to see more sidebars

Summary: First step for sidebar list modal, display button if there are more sidebars, currently clicking button will not do anything

Test Plan: Check if button is displayed correctly, check if it's changing style when unread/read sidebars change

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: zrebcu411, Adrian

Differential Revision: https://phabricator.ashoat.com/D503