Fix custom emoji compatibility in Misskey's display name

[noellabo]

In Misskey, you can use custom emoji in display_name without whitespace, but in Mastodon it will be displayed as a shortcode.

To remedy this incompatibility, insert zero width space before and after the shortcode. Only the shortcodes included in the emoji tag are targeted.

(There are no changes to Mastodon's rules)

[ClearlyClaire]

The need for a space (zero-width or otherwise) between two custom emoji is to accommodate things like IPv6 addresses in messages, so I'm a bit uneasy with this change for this reason. On the other hand, the fact that this only applies to shortcodes that are present in the remote server's payload should make it an exceedingly rare issue though. That being said, if we do that for display names and not for toot content, we introduce a weird inconsistency.

[noellabo]

Ah, IPv6!

You'll find it strange that it doesn't convert Toot content, but Misskey itself inserts whitespace, so we didn't have to deal with it here.

If we limit ourselves to display names, we don't need to be able to parse IPv6 addresses correctly, and it seems to me that we can ignore the rare cases of incorrect conversion.

[stale]

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