Use wiki.disk_path

What does this MR do?

Changes most sites in CE to use project.wiki.disk_path instead of project.disk_path + '.wiki'

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

I left the hashed storage migration uses alone, since this code is fairly tricky and has EE dependencies. I can revisit it if desired?

EE MR needs doing as well.

Why was this MR needed?

DRY. We're storing up trouble by repeating this logic in lots of places.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

CE part of #37391 (closed)