This partially reverts eec6a270.

The problem is that when a conference is protected by a password or has
auth enabled, the "joined" event will come later, when the user
authenticates, so the 10s timeout will kick in.

This is not ideal because we react to the onLoad event of an iframe,
which doesn't tell us much of *what* is actually loaded, but a new event
would be required to properly fix this.

This is the new syntax for extending components. See:
https://github.com/styled-components/styled-components/issues/1546

Wait until the joined event.

Also, create a couple of auxiliary functions so we save some indentation
in the main render function.

The entire page is draggable, except:

- buttons
- any input type
- the filmstrip
- the chat panel

The spinner will be visible until jitsi-meet iframe is completely loaded.

The configured server URL will be used when the room URL is not
a full URL. If a full URL is given, the target server is joined.

The default server URL is https://meet.jit.si

Use the concept of a "server URL", since it's URLs we are dealing with,
not actul domains.

Fixes: #65 

The term "action" is quite overloaded since we use redux,
so let's call it "button" since that's how it behaves.