Shashlik Fixes -> re-add ECAL driven seeding + split off mustache photons

This PR turns the ECAL driven seeding for Shashlik (and HGCAL) on again. 
It was somehow turned off in a few previous patches.

The photons collection is now split between "photons" (old clusters reco) and "mustachePhotons" which uses the output of the mustache super clustering. Only the latter contains photons in the shashlik device.

This can be put in a patch release since it only edits .cc and python.

@nancymarinelli @dbenedet @matteosan1