[web] Introduce DisconnectedBar

Summary:
Introduced DisconnectedBar in web app.
It used the same logic as mobile app.

It had difficulties to correctly style and position the element, but I assume the styles will be mostly changed soon, so the current styles can be considered as temporary

Test Plan: Tested the bar with different network conditions - also with different ping values. It worked in the same way as the mobile one.

Reviewers: palys-swm, benschac, atul, ashoat

Reviewed By: palys-swm, benschac, ashoat

Subscribers: ashoat, Adrian, atul, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D2850