Release 1.12.8

New stable release, with bug fixes and a few featurerettes:

- documentation improvements
- removal of the old threading option and cleanup
- 'lazy' indexing  is even lazier (faster)
- make references (the References:/In-reply-to: headers) searchable
  (with "references:" or "ref:"
- add new combination-field "related:" to search by msgid: _or_
  references:

In mu4e (see docstrings for details):
- new command 'mu4e-view-jump-to-mime-part', to jump to some mime part
by number.
- new variable mu4e-trash-without-flag, if set to non-nil, "trashing" a
message will _not_ add the "T" flag.
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