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`.
