Return 410 code when a question is deleted

[andyvan-ph]

Problem

• When questions get deleted for whatever reason, that URL becomes a 404. Over time, it'll generate a lot of 404s, which I'd like to avoid.

Solution

• If a question is deleted, we should return a 410 status, which indicates the page no longer exists and won't be replaced.

[andyvan-ph]

Updated this issue @smallbrownbike as it was somewhat out of date. Note at all urgent, but would be sensible thing to do when you get a spare moment.