- new config option `singleton`.
- new plugin `converse-uniview`
- removed `converse-embedded`.
- various CSS changes, to properly render an embedded full app
- don't re-open cached and non-autojoined chats in singleton mode

The goal here is to extend the `embedded` `view_mode` so that the full app can
also be embedded, not just a single MUC or private chat.

To do this, we'll need to differentiate between multi and singleton chat apps.

* A singleton chat app contains only a single chat.
* A multi-chat app can contain zero or more chats

So we introduce a new config option, `singleton`, which when used with
`view_mode` set to `embedded` will determine whether a single chat or the full
app is embedded.

Similarly, in `overlayed`, `fullscreen` and `mobile` view modes, `singleton`
set to true will allow only one chat within the parameters of that view mode.

We're appropriating the word `singleton` and introducing the concepts of
`uniview` and `multiview` (see a785ca8) to cover what was
previously meant with `singleton`.

updates #1297