When using `auto_login` and logging out, Converse gets stucks with an infinite spinner.

- 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

Since recently these tests have been failing consistently on Travis,
even though they're passing locally.

Ran out of time debugging the issue, so disabling some stuff for now.

Also update tests

Allows for simpler code, especially making `getReservedNick` easier to override

`initializeDisco` already gets called upon reconnection which
accomplishes this.

- Add `onReconnected` method for chatboxes to clear messages
- Move MAM models to headless build.
- New event `onChatReconnected`

- Save date for older message versions and display it in the modal
- Properly handle the correction being received *before* the corrected message

Fixes #1577

Otherwise we can get into a situation where Converse stops trying to
reconnect.

so that they're cleared when the last tab closes