Regular users should have a way to report spam/abusive posts

[shakesoda]

There's mark as spam for admins, sure, but it'd be nice to be alerted by regular users instead of needing to remain ever-vigilant.

Thinking this would go: user clicks report -> fills in a short memo/selects a category -> an email is sent to the admin -> [maybe] some other action is taken against the user, like rate-limiting for some amount of time until an admin can dish out actual punishment or handle it as a false positive.