now that messages appear in reverse order due to flexbox

By using `display: flex` and `flex-direction: column-reverse`, the chat
now automatically scrolls down when loaded, without requiring any
extra JavaScript.

We still need to scroll down with JavaScript when sending a message.

By using `column-reverse`, the messages container now works in reverse.
So the newest message is the first element in the container and the
oldest message is the last. This is the reverse of before.

Due to this, this change will likely break some plugins.

It gets populated again upon MUC reflection or chat marker

Up and down arrows only serve as message correction shortcuts if the
textarea is empty.

This means that:

- Up arrow doesn't start correction of previous message if we're currently writing a message
- Down arrow doesn't erase currently written message

This is based on user-feedback and is also how Slack handles these
shortcuts.

for limiting messages to a certain number of characters.

For some reason the latter causes extreme lag when switching rooms

Removed one occurrence

When reconnecting with a new transport, we call setUserJID
so that a new resource is generated, to avoid multiple
server-side sessions with the same resource.

We also call `_proto._doDisconnect` so that connection event handlers
for the old transport are removed.

- Don't ever resume SMACKS stream when using BOSH
- Clear SMACKS session data when using BOSH

https://xmpp.org/extensions/xep-0198.html#resumption

The <resume/> element MUST include a 'previd' attribute whose value
is the SM-ID of the former stream and MUST include an 'h' attribute
that identifies the sequence number of the last handled stanza sent
over the former stream from the server to the client.

- Set roster_fetched flag when we get an empty IQ[type="result"] stanza
- Set `restored` flag when resuming in converse-smacks
- Don't trigger change event when fetching xmppstatus values
- Removed two events, since we can just listen to `change` on `_converse.xmppstatus`.

- Test for info message indicating a configuration change
- Test that the MUC reconnects when a `not-acceptable` error is returned

instead of on the view.

Other types of messages such as `error` don't necessarily have a JID
tied to them.

Also, remove ephemeral messages after 10 seconds, not 20

Use-case is when an invalid MUC gets opened in singleton mode.
Without this fix a new chat with spinner appears, even though it
shouldn't.

updates #555

- Remove the `keepalive` configuration setting. It is now always implicitly `true`.
- Remove the `expose_rid_and_sid` configuration setting.
- A `prebind_url` is now mandatory when setting `authentication` to `prebind`.
- It's no longer possible to pass in `rid` and `sid` values to `converse.initialize.