- Add a new method to check for dupes based on the message text.
- When checking for dupes based on origin_id, no need to restrict to
  only our own.

Since those commands aren't only about moderation

Ideally Snabbdom wouldn't completely replace the input when we add the
`error` class, because that's what's causing the input value to be
cleared.

That way we don't lose the user's input values, we avoid flashing and we
avoid unnecessary rendering.

In the process, fixed an annoying issue where Chrome auto-completes
what it thinks is the username into the "Language Tag" field of the MUC
config form.

Instead we tell Chrome that the MUC JID is the username, thereby also
letting it save the password to to that JID.

Looks like the `webkitTransitionEnd` event doesn't fire because `fade`
is already on the modal, so there's no transition, which means that in
bootstrap.native the event doesn't get fired to remove the overlay.

- Let box_id start with char for valid HTML.
- No need to use SHA1 for box id
- No need for the user_id attribute.
- Set nickname when we set the roster contact.

Also...

- _converse.api.contacts.get is now async
- _converse.api.chats.create is now async

File has moved to ./headless/ subdir

Removed "an" in "When writing an a block such as an if or while statement,"

since the next release will be a major one.

This changes the API method's current behavior by not automatically
maximizing (in `overlayed` view mode) or bringing a background chat into
foreground (in `fullscreen` view mode). Instead `force` needs to be set
to `true` for that to happen.

Otherwise we can't enable it during operation

in overlay view mode.

Also refactored a bit to put the method, which determines whether a
chatbox may be shown, on the chatbox itself.