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.
This was in flyte's design (he was following discord) but doesn't feel
too correct when it's not aligned with anything else. We can revisit
this if/when we have better glyphs to represent channel types.
