Updates the `DaySeparator` component to have separately settable colours
for the text and the lines.
Updates existing overrides of the `DaySeparator` to use the new colour
setter.
Create new `ChatOverlayDrawableChannel` with adjusted spacing and new
`DaySeparator` colours.
Reference design: https://www.figma.com/file/f8b2dHp9LJCMOqYP4mdrPZ/Client%2FChat?node-id=1%3A297
Adds new component `ChatTextBox`.
Exposes `BindableBool` `ShowSearch` to change text input behaviour
between normal and search behaviour.
Adds new component `ChatTextBar`.
Exposes `BindableBool` `ShowSearch` which toggles between showing current
chat channel or search icon.
Additionally binds to child `ChatTextBox` components.
Requires a cached `Bindable<Channel>` instance to be managed by a parent
component.
