Messenger style issues

Summary

This MR changes the form toast container so it only renders when there's visible toasts and fixes the z-index issue that caused the navigation sidebar to appear on top of the conversations

Closes #3133 (closed)

Steps to test

1. Open a messenger conversation, make sure the window is ~1350px wide
2. You should be able to type a message
3. Set the window's width to 565
4. The conversation should appear on top of the sidebar

Estimated Regression Scope

There's the remote possibility that it could break form toasts, although I've tested it and it works fine