[ticket/12674] Use user's colour in 'Last edited' line in sent PMs

PHPBB3-12674
Merge remote-tracking branch 'Elsensee/ticket/12674' into develop-ascraeus

# By Oliver Schramm
# Via Oliver Schramm
* Elsensee/ticket/12674:
  [ticket/12674] Use user's colour in 'Last edited' line in sent PMs