Allow flags to be retracted

[cellio]

https://meta.codidact.com/posts/281843

If you flag something by mistake, or flag and change your mind, we don't currently have a way for you to retract that flag. I see a status-planned on that post but I can't find an issue, so I'm filing this one -- if it's a dupe please go ahead and close as such.

The way this could work is: if you select "flag" and you have a pending flag already, it would be highlighted somehow and there would be a "retract flag" button. If you select a different flag (it appears we let you flag more than once), then the "retract" button would go away and you'd have the regular flag interface.

There would also be a "retract flag" button, for still-pending flags, on the flags list reached from the user profile.

[cellio]

Since this issue was filed, we've modified the flag interface to notice if you have a pending flag and ask if you want to flag again. So half the work is done; maybe it's easier to add a "retract" button there?