Another hack at top of page test-group-page signatures

Changing the code stamped onto the top of pages by plom (for test-pages). Is now

1234.ggg pp

where ggg is the group to which the page belongs 'id' or 'dnm' or 'q7' (say), and pp is the page number 'p3' (say)

To make this work we have to pass around a "page to group" dictionary that maps a page number to the name of the group to which it belongs.

Needs

• decision = is this format good enough
• decision = what do we do for user-modified labels for questions (how do we handle weird question-names?) Defer to #1886?
• decision = do we move the page_to_group function into the spec-verifier code (probably we should)
• decision = what else in this list.
• changelog entry

Then - mergin.