The previous change in 1871b57663 failed
to consider the browser refreshing case, as reported here:
https://forums.ankiweb.net/t/anki-2-1-50-beta-3-4/17501/30
I previously attempted to solve this by having SetFlag skip the queue
rebuild, then mutating the captured mtimes in the queues. That didn't
work correctly when undoing, as the queue mutations weren't recorded.
This approach combines that attempt and the previous change: flag
setting is an undoable operation again, but does not change the card's
modification time, so it can be applied/undone without a queue build
being required. Instead of special-casing flag changes in the review
screen, we now just redraw the flag on changes.card, as any other card
op will have triggered a queue rebuild.
d52c36e920