hwscan multiquestion: allow pushing hwpages to multiple questions

Add hwscan process foo_12345678._.pdf 12345678 -q 1,2,3.

This creates multiple Hpage entries from the same image (only one image is created in the database).

TODO list

• This seems to work but looks poor in PageTwister (there are 3 copies of very page: h.1.n and h.2.n and h.3.n for all n). Not clear this is "right" way or whether Loose pages can be improved instead. Fixed on master branch, client-side, along with many other improvements to adjust-pages driven by this.
• should work if you pass -q 1,2 on a 4-question paper: allows "Omeric-style" interleaving.
• in some places in the code I called this "semiloose": maybe that should be removed. update: who cares.
• not clear what a use case for Loose: the Loose vs HWPage seems to rigid. Like what can Loose do that -q 1,2,3,4 can't? (assuming 4-question paper). File new issue to remove Loose: #1309
• factor out the qref checking in db_upload from the loop.
• changelog entry
• update help text in hwscan.
• support literal all: -q all to reduce human error in common case of "upload every page to every question". Filed #1308 (closed).