Internal: Use Contact::override_deadlines().

In preference to privChairOverride().
Internal: Introduce Contact::override_conflict() function.

Rather than a bunch of duplicated $_REQUEST['forceShow'] logic.
Internal: Prepare for per-paper administrator status.

Use $Me->canAdminister($prow) instead of $Me->privChair, even though
for now Contact::canAdminister($prow) equals $Me->privChair.

Sarita Adve would love per-paper administrators to help handle chair
conflicts, although honestly I'm not sure it's a good idea.