Tagger: Introduce helper class for tags.

Take the opportunity to clean up some API issues.
Internal: PaperList passes fields around in preference to fieldIds.
PaperList::showHeader defaults to don't show header.
Consolidate unused field and list types.
Paper list: Each field gets a unique name.
Internal: Add helper Qobject class for quiet objects.

Constant use of "defval()" to avoid warnings on unset properties grows
tiresome.
Internal: Use Qobject to simplify information export from PaperList.
Internal: Refactor paper columns so they render themselves.
Internal: Refactor paper columns so they sort themselves.
Internal: Remove $paperListFolds global in favor of per-field information.
Internal: List contents are field names.
Mark a TODO.
Internal: Remove field IDs from consideration.

Push field objects through sorting and special sorting. Also allow
sorting by a field that's not displayed.
Internal: More column rearrangement.

This checkin also cleans up some permissions---we are careful about
whether a user can view a particular reviewer *at display time* as well
as search time. There were no bugs, I think, but this is cleaner.

Minor functionality loss: a search for 're:lily' no longer highlights
Lily's scores in the score graphs.
Internal: Reduce inconsistency in $forceShow API.

$forceShow, recall, is the "override conflicts" button on papers with
which the chair is conflicted. In some situations the "override
conflicts" setting should be ignored. The APIs for obeying $forceShow
were inconsistent, which led to some minor bugs. Make them more
consistent: $forceShow === null means check $_REQUEST; otherwise treat
$forceShow as a truth value for whether to force.
Nitlet: New session lists contain q= parameters whenever they should.
Nitlet: Correctly handle PaperConflict rows with conflictType<=0.
Internal: For consistency, author queries also define a null reviewType.
Internal: Complete refactoring of list columns into smarter objects.

Each object knows whether it's allowed to render for a given user.
Which would make it safe for a user to request a named column (it
wasn't safe before).