Also add the ability to disable MAM for certain chats (such as the headlines chat).

updates #622

One visible effect is that when this component is now removed,
minimize buttons are not visible on the chat boxes.

updates #622

via the new config option credentials_url

Don't call trimChats in onChatBoxAdded event. Reduntant because it will be
called after maximization and after being shown.

Add new method getShownChats and don't trim if only one or zero chats are being
shown.

Don't trim chats when in responsive mode.

Don't call trimChats unnecessarily in the render method of the trimmed chats
thingy.

fixes #437

skeleton-sass doesn't appear to be used.
otr.js is not a dev dependency, while sinon is.

- Don't trim if not connected.
- Move trimChats call to converse-minimized
- Immediately hide view to avoid race conditions.

Previously views were only hidden, but not removed. This was an unnecessary
"optimization" which introduced unnecessary complexity.

Problem solved was that closing minimized chats didn't dereference the model
for hidden "normal" chat views, causing an exception to be raised when trying
to log out.

Couldn't handle messages with no "from" attribute.
Some refactoring to add code that checks if a messages is a headline to the
utils module.
Updated tests. Add sinon so that we can test returned value of spy.

Remove split of fullname. Fixes #609

Fixes #156

Instead we use event handlers.

Had to add a `reconnect` event to core.

Also generated new dist files.

updates #616