Revise the deprecation logging

This introduces a more automatic way of increasing the log levels for deprecation log statements based on the version it was deprecated.

The thresholds are a little arbitrary, but

* We log INFO for 6 releases
* We log WARN for another 6 releases
* THen ERROR (failing the build)

This should give theme authors plenty of time to catch up without having the log filled with warnings.

hugolib/hugo_sites_build.go

@@ -25,7 +25,6 @@ import (
 
 	"github.com/bep/logg"
 	"github.com/gohugoio/hugo/hugofs/files"
-	"github.com/gohugoio/hugo/langs"
 	"github.com/gohugoio/hugo/publisher"
 	"github.com/gohugoio/hugo/tpl"
 
@@ -40,14 +39,8 @@ import (
 	"github.com/gohugoio/hugo/output"
 
 	"github.com/fsnotify/fsnotify"
-	"github.com/gohugoio/hugo/helpers"
 )
 
-func init() {
-	// To avoid circular dependencies, we set this here.
-	langs.DeprecationFunc = helpers.Deprecated
-}
-
 // Build builds all sites. If filesystem events are provided,
 // this is considered to be a potential partial rebuild.
 func (h *HugoSites) Build(config BuildCfg, events ...fsnotify.Event) error {