Preview new files before committing them

What does this MR do and why?

Implements Add preview for new markup files (#15501), a decade-old feature request.

Add the same Write and Preview tabs the edit page has to the new file page.

The edit page's preview machinery carries over as is: the renderer is picked from the file name in the editor (Follow mid-edit renames in the preview renderer (!251790 - merged)), and the live preview follows the Markdown extensions currently loaded (Follow mid-edit renames to Markdown in the "Pre... (!250889 - merged)), so the frontend (edit_blob.js) needs no changes.

That leaves the view and the controller: add the tabs to the view, and make the preview action, which assumed a committed blob, work without one (new files, empty repositories).

Build on

• "Preview" tab does not follow mid-edit file ren... (#616115 - closed)
  1. Cover mid-edit renames from Markdown in the "Pr... (!253126 - merged)
  2. Follow mid-edit renames to Markdown in the "Pre... (!250889 - merged)
  3. Follow mid-edit renames in the preview renderer (!251790 - merged)
  4. Fix races between Markdown extension loading an... (!251306 - merged)

Screen recordings

Manually verified in GDK-in-a-box:

How to set up and validate locally

1. Open a directory in a repository and select + > New file, then enter * Now you see me as the content
2. Name the file README.md and click Preview — the Markdown live preview opens
3. Rename it to README.org and click Preview — the content renders as an Org-mode heading
4. Rename it to README.txt and click Preview — a diff with every line added

Notes

• Backend engineer, but fairly new to Rails. Feedback on Rails conventions welcome.

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