This new navbar component is used for every navbar in chat, full page or drawer, and any screen.
This commit also uses this opportunity to correctly decouple drawer-routes from full page routes. This will avoid having this kind of properties in components: `@includeHeader={{false}}`. The header is now defined in the parent template using a navbar. Each route has now its own template wrapped in a div of the name of the route, eg: `<div class="c-routes-threads">..</div>`.
The navbar API:
```gjs
<Navbar as |navbar|>
<navbar.BackButton />
<navbar.Title @title="Foo" />
<navbar.ChannelTitle @channel={{@channel}} />
<navbar.Actions as |action|>
<action.CloseThreadButton />
</navbar.Actions>
</navbar>
```
The full list of components is listed in `plugins/chat/assets/javascripts/discourse/components/navbar/index.gjs` and `plugins/chat/assets/javascripts/discourse/components/navbar/actions.gjs`.
Visually the header is not changing much, only in drawer mode the background has been removed.
This commit also introduces a `<List />` component to facilitate rendering lists in chat plugin.
