Emoji: First pass at support in Interactions

[obenland]

Heads up: This PR adds img tags to the list of allowed HTML tags for Interactions!
Should we sideload emoji images?

Fixes #970.

See https://mastodon.social/@obenland/113788023027390776
See https://obietester.blog/2025/01/06/99/#comment-31

Proposed changes:

• Adds callback function that replaces custom emoji strings with their image representations when available.
• Adds unit test to cover the new function.
• Runs author name and comment content through the new callback on insert and update.

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Go to Mastodon, for example, and reply to a federated test post with a custom emoji. :yikes:, :AngeryCat:, etc.
• Go to the comment list in wp-admin and make sure the emoji render as images.

[pfefferle]

I think before allowing images in comments, we have to think about/implement a proper blocking and moderation tooling (even more if it is about side-loading images): https://www.theverge.com/2023/7/24/23806093/mastodon-csam-study-decentralized-network

[obenland]

I just pushed an update that moves the replacement into a filter that runs after comment_content and the author have been sanitized. That way, only custom emoji images will be added to the content.