refactor wiki page edit locking

  - Adds a new function in 'inc/pageutils.php',  wikiLockFN($id)
  - All page edit locks should now be created with the file name
    generated by wikiLockFN($id).
  - wikiLockFN
    - Generates wiki page editing locks in the 'data/locks'
      directory where they belong.
    - This avoids polluting the 'data/pages' directory with lock files,
      which were causing namespaces to be created before they logically
      should exist.

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