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

config/utils.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/mattermost/mattermost-server/v5/mlog"
 	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v5/shared/i18n"
 	"github.com/mattermost/mattermost-server/v5/utils"
 )
 
@@ -110,7 +111,7 @@ func fixConfig(cfg *model.Config) {
 func FixInvalidLocales(cfg *model.Config) bool {
 	var changed bool
 
-	locales := utils.GetSupportedLocales()
+	locales := i18n.GetSupportedLocales()
 	if _, ok := locales[*cfg.LocalizationSettings.DefaultServerLocale]; !ok {
 		*cfg.LocalizationSettings.DefaultServerLocale = model.DEFAULT_LOCALE
 		mlog.Warn("DefaultServerLocale must be one of the supported locales. Setting DefaultServerLocale to en as default value.")