Import image markdown for Github Enterprise Self-Managed
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When working on !130688 , it was realized copying markdown attachments from GitHub Enterprise **Self-Managed** does not work.
Per: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/130688#note_1547178663, importing GitHub Enterprise Self-Managed image markdown was never tested and should never have been working.
This is because the self-managed urls can be different depending on the object storage chosen. Eg. An example GCE URL would look like [https://35.231.21.147/storage/user/4/files/a58ddef5-9601-4740-ad25-686a2e0da3f](https://35.231.21.147/storage/user/4/files/a58ddef5-9601-4740-ad25-686a2e0da3f) , which does not follow any url pattern matching we have implemented for scraping Github.
Note: This is different than GitHub's Enterprise SaaS offering, as that works the same as regular GitHub.com
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