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.
