Migrate EmojiFilter to use native support

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comrak supports emoji parsing natively. By leveraging that, we may be able to remove our own EmojiFilter. This would boost performance and security.

Currently we add several attributes to an emoji. For example, 😄 becomes

<gl-emoji data-unicode-version="6.0" data-name="smile" title="smiling face with open mouth and smiling eyes">😄</gl-emoji>

We would probably need to still add those attributes using a filter. Though it's possible we could do this by using our own HTML output method in the GLFM Markdown gem. That is still under investigation.

This would not remove the need for our CustomEmojiFilter. Though if unsupported emojis could be tagged in the rust parser (not possible today), we could more easily find these in the HTML and resolve them.