[web] Add tooltip to open existing sidebars

Summary: First part of creating sidebars on web: adds tooltip to multimedia and text messages when sidebar exists and allows to open it

Test Plan: Check if tooltip is displayed for messages that started a sidebar and if pressing it opens correct sidebar.

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: zrebcu411, Adrian, atul, subnub, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D708