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.

This is necessary for cleaning up after tests (now that each test has its own
initialized converse).

Should hopefully not cause any issues when logging out during normal use.

and don't show the online count in the toggle.

Otherwise RosterContactView objects stay around and keep on listening.

Only a small subset of tests currently working with this.