Fixes #1262. Redirect to issue page after bug creation.

[miketaylr]

Let's try this rather than #1272.

r? @karlcow

[karlcow]

r+ @miketaylr

[miketaylr]

Thanks @karlcow -- mind explaining how you might do this via Referer?

[karlcow]

My naive thinking…

POST /issues/new

which creates a redirect to /issues/[number] could send along that the Referer is /issues/new.
When this Referer is there you know you need to send the thanks flash.
when someone does a direct query to issues/[number] there's no referer with /issues/new value.

[miketaylr]

Ah, that's kinda cool. Using session feels less elegant, I'm not sure if it is or isn't.