Add dummy channel `DummySelectorChannel` which should be set as the
current channel in the channel manager when the selector in the chat
overlay should be shown.
Refactors the `ChannelListItem` to not show mention pill and close
button when the channel is the dummy selector channel.
Ensure that the `ChannelList` selects the dummy channel on clicking the
selector item.
Removes `ChannelListSelector` as it is no longer needed.
Removes the `setCurrent` parameter from `ChannelManager.JoinChannel`
method as it is no longer needed.
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.
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).
