Refactors `ChatLine` component to use more sensible override properties
and layout using grid container. Moves creation of username component
into its own method to simplify BDL.
Updates padding of base `DrawableChannel` flow padding.
Removes usage of `ChatOverlayDrawableChannel` since it's overrides are
no longer needed.
Updates usage of `StandAloneChatDisplay` to use new override properties
of `DrawableChannel`.
Moves `DaySeparator` chat component to it's own file and update it to
match new chat design. Makes use of several virtual attributes that can
be overridden to update spacing and layout in other usage contexts.
Remove redundant usage of `ChatOverlayDaySeparator`, since the new
design is now part of the base class.
Create `StandAloneDaySeparator` to use in `StandAloneChatDisplay` which
overrides attributes to match correct spacing and layout for its design.
Ensure that `DrawableChannel.CreateDaySeparator` returns type of
`DaySeparator` instead of `Drawable`.
