[web] [remove] [ENG-1022] add long arrow and implement in sidebar

Summary:
remove current implementation, that's the icomoon icons + border-left extensions.
Add -- a longer svg arrow for Chat TheadItems that have more than one sidebar.

This is just to add the second arrow for a Chat Thread Item with more than one sidebar.

Note:

• ENG-1053 added updated arrow here
• I still need to update the ChatThreadItem styling a bit outlined here: ENG-1073

Test Plan:
Icons should be completely removed. The design is pretty close to figma: https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/Comm-%2F-Desktop-app?node-id=1170%3A79456

The ChatThreadItems height needs to be adjusted a bit more.

Reviewers: atul, palys-swm, def-au1t, yayabosh

Reviewed By: palys-swm, def-au1t

Subscribers: ashoat, Adrian

Differential Revision: https://phabricator.ashoat.com/D3839