Handle new Confluence cloud ac:emoticon tags in which ac:emoji-fallback doesn't contain an actual emoji anymore

See https://forum.xwiki.org/t/confluence-import-broken-emoticon-import-with-new-confluence-format/16354

Formerly, we could rely (or at least we thought we could rely) on ##ac:emoji-fallback## containing the actual emoji, like in the following:

<ac:emoticon ac:name="smile" ac:emoji-shortname=":slight_smile:" ac:emoji-id="1f642" ac:emoji-fallback="🙂" /

(and, by the way, on ac:emoji-id containing the hexa code of the emoji)

Now we can't, as code like this exists:

<ac:emoticon ac:name="cross" ac:emoji-shortname=":cross_mark:" ac:emoji-id="atlassian-cross_mark" ac:emoji-fallback=":cross_mark:" />

We need to rely on ac:name first, and only fall back on other options if we don't know the emoji.