updates #1220

Fixes #1220

* Add support for webm video and m4a audio media files
* updated CHANGES.md

They're no longer needed since we're using webpack

Silently set status when it comes from a presence stanza

Otherwise the private chatbox header doesn't show in Firefox

Update to Strophe 1.2.16

They're not included in the repo and this makes it more difficult for
people to try Converse locally

Previously the Promise.all made it all-or-nothing, now it'll return
partial matches as well and it'll also log error stanzas.

Also fix if/boolean bug when checking whether we should automatically
register

This is to avoid having a stale/inaccurate MUC features list when
reopening a previously closed MUC

in which case a disco query is not sent out again because we still have
it cached.

This will cause their nick to be registered (if Prosody supports it).

Also expand and fix the associated test.

By doing so we create a new DiscoEntity for the room, which allows us to
query for features via the disco API.

We also avoid duplication of functionality between converse-muc and
converse-disco