"Report this" feature

[EventideGlow]

Suggested here.
In part, this is a stepping stone to user-curated tags with #23.

A "report this" button should be placed on all types of content on Pony.fm so that users can report anything from a mis-tagged song to unmarked explicit content or any other issues to a site admin.

We can get a useful v1 of this out the door very quickly by making this a glorified "contact us" form that allows the user to enter freeform text for their report, then recording the report in the database and emailing out a copy of it.

Other features that can be added after v1 is deployed to make this even more useful:

• an area visible only to staff that displays reports and allows staff to comment on them and mark them resolved
• integrating reports into the notification system (this may be doable as part of v1)
• pre-defined "common report reasons" to make it easier for users to submit simple reports
• integrating reports more tightly with content using a polymorphic relation (a Reportable interface?)