R1:db69686927bd

Make pressing "R" on your keyboard reload the card state on workboards

Summary:
Depends on D20638. Ref T4900. This is an incremental step toward proper workboard updates.

Currently, the client can mostly update its view because we do updates when you edit or move a card, and the client and server know how to send lists of card updates, so a lot of the work is already done.

However, the code assumes we're only updating/redrawing one card at a time. Make the client accept and process multiple card updates.