Support stack new

[ndmitchell]

"stack new" fails in both cases with:
HttpExceptionRequest Request {
host = "raw.githubusercontent.com"
port = 443
secure = True
requestHeaders = [("If-None-Match",""8ef96691e15d68bb890626821e8624b1b19e0b6c"")]
path = "/commercialhaskell/stack-templates/master/new-template.hsfiles"
queryString = ""
method = "GET"
proxy = Nothing
rawBody = False
redirectCount = 10
responseTimeout = ResponseTimeoutDefault
requestVersion = HTTP/1.1
}

[ndmitchell]

As far as I can tell, the URL/path are hardcoded, see https://github.com/commercialhaskell/stack/blob/master/src/Stack/New.hs, in particular defaultTemplateUrl and similar.

[ndmitchell]

Known issue, raised at commercialhaskell/stack#2804.