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.
Reference design: https://www.figma.com/file/f8b2dHp9LJCMOqYP4mdrPZ/Client%2FChat?node-id=1%3A297
Adds new component `ChannelList` which makes up the sidebar channel
selector of the new chat design. Contains two separate fill flows for
public and private `ChannelItem` child components.
Exposed bindable `SelectorActive` to indicate current state of the "Add
more channels" button. Requires `Bindable<Channel>` from parent
component.
Renames and updates the `TestSceneChannelListItem` to
`TestSceneChannelList` to make use of new component and having both
tests seemed redundant.
