Import image markdown for Github Enterprise Self-Managed

<details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=424400) </details>  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

issue

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