Extracting i18n functionality to i18n core library (#16914)

* extracting i18n functionality to i18n core library

* Removing utils.T

* Adding documentation and changing one function name for better explanation

* Changing other missing utils.T

* Adding license string

* Renaming corelibs to pkg

* Renaming corelibs to pkg (moving directory)

* Renaming from pkg to shared

* Fixing bodyPage.Html casing

* Fixing merges

* Fixing merge problem

* Fixing tests

app/layer_generators/opentracing_layer.go.tmpl

@@ -9,7 +9,6 @@ package opentracing
 import (
 	"github.com/opentracing/opentracing-go/ext"
 	spanlog "github.com/opentracing/opentracing-go/log"
-	goi18n "github.com/mattermost/go-i18n/i18n"
 )
 
 type {{.Name}} struct {