Avatar didn't update a second time due to `canvas` element being expected

Otherwise we run into a bug where two tabs with Converse.js share the
same XEP-0198 SM-ID, causing both to go into a reconnection-loop as
the XMPP server switches XEP-0198 sessions between them.

This bug is due to a distinction in how sessionStorage behaves when you
open the existing site in a new tab (e.g. middle-click or
`target="_blank"), as opposed to creating a new tab
and then opening the site in that tab.

In the latter case, the newly created sessionStorage object is empty.
In the former, the contents of sessionStorage of the current page is
copied over to the new page!

otherwise a new cache entry gets created, causing multiple messages to
be restored from cache later on.

Distinguish between getting an existing entity and
creating a new one.

When creating a new one, ensure that we don't fetch from the cache.

New API method for creating a disco entity.

Only fetch messages after we have the latest room features
Otherwise we run into race conditions where MAM messages are fetched
before we know whether (updated) the room supports MAM or not.

I'm not sure how much this is an issue outside of tests, where we might
run into race conditions arising to the fact that we don't always
respond to every IQ stanza

"- Properly handle message correction being received before the corrected message"
was there twice

- Fixes #1575. Height not properly set on MUC invitation list
- Add new configuration setting: `muc_mention_autocomplete_min_chars`
- Bugfix. `min_chars` setting for MUC mentions wasn't having an effect

Unfortunately this doesn't work on Firefox and there's no proper
workaround. https://github.com/philipwalton/flexbugs/issues/108

Reverts:

Revert "Bugfix. Properly insert error messages and spinner"
This reverts commit 6a419cc1.

Revert "Use flexbox to keep the chat scrolled down"
This reverts commit dd91d3cc.

when the textarea contains an unsent message

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