Implement linked file

The Linked file feature should support:

  1. Placing the linked diff file first in the list
  2. Linking to expanded lines
  3. Preserving MR version
  4. Working as fast as possible (potentially needs to switch to a file path-based identifier)
  5. Compatibility with caching (linked file should don't be repeated in the list)
  6. URL updates reflected in email links
  7. Backwards compatibility with legacy URLs (just scrolling to a file)
Edited by Stanislav Lashmanov