Update Banzai markdown processor to properly group the skin tones

Summary

Following the discussion on Gitter, emoji rendering going through the Banzai markdown processor splits the skin tone into separate <gl-emoji> tag.

Before markdown rendering

👍🏿

<gl-emoji title="thumbs up sign" data-name="thumbsup" data-unicode-version="6.0">👍</gl-emoji><gl-emoji title="emoji modifier Fitzpatrick type-6" data-name="tone5" data-unicode-version="8.0">🏿</gl-emoji>

:thumbsup_tone5:

<gl-emoji title="thumbs up sign tone 5" data-name="thumbsup_tone5" data-unicode-version="8.0">👍🏿</gl-emoji>

After markdown rendering

👍🏿 👍🏿

👍🏿

---

:thumbsup_tone5: 👍🏿

👍🏿

Possible fixes

See /lib/banzai/filter/emoji_filter.rb and /spec/lib/banzai/filter/emoji_filter_spec.rb.