Specifically the methods related to requesting an upload slot and uploading a file.
Also show a progress indicator while a file is being uploaded.

Updates #161

It contained only `overrides` and some HTTP upload code was in other
modules.

Current thinking concerning overrides:

Usage of `overrides`, while useful in certain cases, should in general
be discouraged, since it's in essence "monkey patching" which makes it
more difficult to know whats executing at runtime and more difficult to
refactor.

Splitting modules up between XEPs is not always that useful. Some XEPs,
like HTTP Upload (and MAM comes to mind) have their functionality spread
out over single and group chats (and pubsub) and might for practical
purposes be considered "core" enough to not try and keep them in
separate modules (which inevitably requires overrides or a fundamentally
rethinking the architecture).

Where splitting code between modules makes a lot of sense is in keeping
Backbone Models and Views separate (so that alternative view libraries
like Vue could be used) and probably in keeping Single chats, MUC,
PubSub and MIX separate.

updates #161

updates #161

updates #161

updates #161

* Use Promises instead of callbacks
* Update to latest (Last Call) version of XEP-0363
* Move non-view specific methods to models instead
* Add more tests

updates #161

Previously we kept all entities and their items (which are also
instances of _converse.DiscoEntity) in a flat array.

Instead, we should have a tree-like structure where items are stored
on the relevant entity (and recursively on other items).

Merge branch 'httpFileUpload' of https://github.com/worlword/converse.js into worlword-httpFileUpload

If "inVerse" is renamed to "fullscreen" then "inVerse" in the fullscreen.html file should be renamed to "Converse"?

This reverts commit 7303e96c.

Instead of dropping the feature entirely, we can check if the CSN is a
carbon message.

used code from 'http-file-upload' branch. Buttons are now added to the toolbars through converse-http-file-upload.js.

To avoid showing them erroneously would mean keeping track of used
message ids which seems like more work than justified by the feature.

Fixes #1026

To more cleanly separate views and models and to make MUC in headless
mode more viable.

Refs #1032

* Distinguish between CSN messages and other types of messages
* Properly clear previous notifications
* Better handling of notifications from multiple users (in MUC)
* Rename methods to make clarify intent