Merge remote-tracking branch 'origin/search'

Merge remote-tracking branch 'origin/search'

* origin/search:
  view: deduplicate common code for loading search results
  SearchMsg: ensure metadata for ghost messages mid
  implement /s/$SUBJECT_PATH.html lookups
  search: remove unnecessary xpfx export
  www: /t/$MESSAGE_ID.html for threads
  view: hoist out index_walk function
  view: reply threading adjustment
  thread: common sorting code
  view: display replies in per-message view
  search: make search results more OO
  extract redundant Message-ID handling code
  search: implement index_sync to fixup indexer
  initial search backend implementation