Adds the logic to render TestHint only when the test mode is enabled
in order to be able to put independent TestHints in other places than
the TestConnectionInfo component.

Adds TestConnectionInfo component which exposes some internal app state
to the jitsi-meet-torture through the UI accessibility layer. This
component will render only if config.testing.testMode is set to true.

The basic indicator is extracted into a LoadingIndicator component,
which then NetworkActivityIndicator displays (or not) based on network
activity.

I'm not saying that the two commits in question were wrong or worse than
what I'm offering. Anyway, I think what I'm offering brings:

* Compliance with expectations i.e. the middleware doesn't compute the
next state from the current state, the reducer does;

* Clarity and/or simplicity i.e. there's no global variable (reqIndex),
there's no need for the term "index" (a.k.a "reqIndex") in the redux
store.

* By renaming net-interceptor to network-activity feels like it's
preparing the feature to implement a NetworkActivityIndicator React
Component which will take on more of the knowledge about the specifics
of what is the network activity redux state exactly, is it maintained by
interception or some other mechanism, and abstracts it in the feature
itself allowing outsiders to merely render a React Component.

Works only for XHR requests, which is the only network request mobile performs
(WebRTC traffic aside). The fetch API is implemented on top of XHR, so that's
covered too.

Requests are kept in the redux store until they complete, at which point they
are removed.