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

web/context.go

@@ -13,6 +13,7 @@ import (
 	"github.com/mattermost/mattermost-server/v5/audit"
 	"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"
 )
 
@@ -87,7 +88,7 @@ func (c *Context) LogAuditWithUserId(userId, extraInfo string) {
 
 func (c *Context) LogErrorByCode(err *model.AppError) {
 	code := err.StatusCode
-	msg := err.SystemMessage(utils.TDefault)
+	msg := err.SystemMessage(i18n.TDefault)
 	fields := []mlog.Field{
 		mlog.String("err_where", err.Where),
 		mlog.Int("http_code", err.StatusCode),