Include full image URL in webhooks for uploaded images

Todo list for Sean

• Fix broken specs
• Move MarkdownUtils.absolute_image_urls to be in the webhook builder class
• Make all hooks here use builder classes
• Add specs for the rewriting
• Add documentation on the limitations, as described in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6031#note_64320839
• Add a changelog entry, crediting @satishperala

What does this MR do?

Rewrites 'simple' image upload Markdown to use absolute, not relative URLs. This will fail for manually-added images, images in code blocks, etc.

Are there points in the code the reviewer needs to double check?

Is the documentation on the limitations sufficient? Or should we try the other way (HTML -> Markdown)?

Why was this MR needed?

It's annoying when you get a webhook with an image path that is relative to the originating server.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/20720.