Member view status on work orders

[tymorrow]

• A member opens a work order
  • A "view" record is added to the database representing that they've read that work order.
• A member comments on or otherwise modifies a work order (might exclude status/priority changes from having this effect)
  • All previous "view" entries in the database for that work order are removed and a new one is added for the modifying member since they're the only one who has read it since the last change.

Members looking at the work order index will then be able to see which work orders contain content they have not read. This would be particularly useful for the House Manager.

Also, add unviewed count to side nav; people need to know when people reply to a work order.

[tymorrow]

Work orders are simplifying a lot in #76 and will obsolete this, so I'm going to have this closed once that task completes.