Move Gitlab::Markdown::* into Banzai

Asherah Connorrequested to merge
kiv/move-gitlab-markdown-into-banzai into master

What does this MR do and why?

Move Gitlab::Markdown::* into Banzai.

See !254944 (comment 3835926392) and Refactor Banzai into `./gems` (#629225). There was no need for this to be in a new namespace, and this is part of the code that should eventually get taken into a decomposed gem.

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