Provides initial implementation of new chat overlay in component
`ChatOverlayV2`. Contains only the basic functionality required for
a functioning chat overlay according to the new design with the intent
of added the rest of the functionality in subsequent PRs.
Backports existing tests for the current chat overlay except for ones
testing keyboard shortcuts (since they haven't been added) and tab
closing behaviour (since no tabs).
Without this change, there would be a couple of pixels between each list
item where nothing would be hovered. This is a pretty annoying UX which
we should be avoiding we possible.
