Since the element will already be taking up space (due to the
`opacity:0` property being used, instead of `display:none`).

Updates #723, #734

`converse.rooms.open` ignored the `muc_nickname_from_jid` setting.

- Reconnection failed if original connection was never established
- When the `credentials_url` is specified, keep on trying to reconnect.

Bug got introduced during fix for #677

Eventually had to do a significant refactoring, to more consistently use the
`hidden` class instead of `display: None`. (relevant for #695)

Can get annoying very quickly

To which it's actually applicable (instead of core).

before populating the roster.

responsible for showing messages based on the extended presence information.

- Use DOM API instead of jQuery
- Make info messages overridable (allows also for disabling them)
- Break up large function with inline callbacks into smaller ones

Via `rooms.open` API method.

together with keepalive.