bug: Not possible to show loading indicator on initial /replies load in Thread

[MartinCupela]

Describe the bug

Thread component does not handle the message list state, when there are not messages loaded yet, but the HTTP request to /replies endpoint is in progress.

Meanwhile the Channel component keeps track of loading state only for the channel.watch call, it does not keep track for the scenario, when the user opens a non-empty thread for the first time since the channel was loaded. Keeping track of channel.watch call allows the integrator to display LoadingIndiciator component. Both main message list as well that of thread should keep of their loading state.

Related to #1635

Note: this will be a bug once theming-v2 is released.