[web] Simplify MessageListContainer and ChatTabs CSS

Summary:
Introduce some CSS improvements as a follow-up diff after D4434.
The app looks the same as before, but absolute positioning and hardcoded 400px margins were removed and replaced with flex, so the code is easier to maintain.
We don't have blue color in out color palette, but I kept existing border color unchanged, but exported into variable.

Test Plan:
Run web app, tested most of the views performing different operations with different zoom options to confirm the view scrolls correctly and looks like before. Tried to attach multimedia to confirm the border looks like before.
Performed tests on Chrome, Safari and Firefox.

Reviewers: tomek, atul, ashoat

Reviewed By: tomek, ashoat

Subscribers: ashoat, adrian, abosh

Differential Revision: https://phab.comm.dev/D4599