Add authentication to markdown API

What does this MR do and why?

Now require user to be logged into GitLab in order to use the Markdown REST API endpoint, https://docs.gitlab.com/ee/api/markdown.html

This can be turned off by disabling the operational feature flag, authenticate_markdown_api

How to set up and validate locally

If you try against gitlab.com, the following should work and return HTML:

curl --header Content-Type:application/json --data '{"text":"Hello world!"}' "https://gitlab.com/api/v4/markdown"

Using your local instance, like this

curl --header Content-Type:application/json --data '{"text":"Hello world!"}' "http://localhost:3000/api/v4/markdown"

you should get {"message":"401 Unauthorized"} returned.

You can disable the feature flag by using Feature.disable(:authenticate_markdown_api, type: 'ops') in the Rails console

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #369369 (closed)