What started as an attempt to fix a bug in parseXUserElement, turned into
another large refactoring of MUC code, and it's not clear how to break this up
into multiple atomic commits. So I'm just pushing it all.

At least there are two new tests added to the suite.

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.