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/admin.go

@@ -16,6 +16,7 @@ import (
 	"github.com/mattermost/mattermost-server/v5/mlog"
 	"github.com/mattermost/mattermost-server/v5/model"
 	"github.com/mattermost/mattermost-server/v5/services/mailservice"
+	"github.com/mattermost/mattermost-server/v5/shared/i18n"
 	"github.com/mattermost/mattermost-server/v5/utils"
 )
 
@@ -205,7 +206,7 @@ func (a *App) TestSiteURL(siteURL string) *model.AppError {
 
 func (a *App) TestEmail(userID string, cfg *model.Config) *model.AppError {
 	if *cfg.EmailSettings.SMTPServer == "" {
-		return model.NewAppError("testEmail", "api.admin.test_email.missing_server", nil, utils.T("api.context.invalid_param.app_error", map[string]interface{}{"Name": "SMTPServer"}), http.StatusBadRequest)
+		return model.NewAppError("testEmail", "api.admin.test_email.missing_server", nil, i18n.T("api.context.invalid_param.app_error", map[string]interface{}{"Name": "SMTPServer"}), http.StatusBadRequest)
 	}
 
 	// if the user hasn't changed their email settings, fill in the actual SMTP password so that
@@ -224,7 +225,7 @@ func (a *App) TestEmail(userID string, cfg *model.Config) *model.AppError {
 		return err
 	}
 
-	T := utils.GetUserTranslations(user.Locale)
+	T := i18n.GetUserTranslations(user.Locale)
 	license := a.Srv().License()
 	mailConfig := a.Srv().MailServiceConfig()
 	if err := mailservice.SendMailUsingConfig(user.Email, T("api.admin.test_email.subject"), T("api.admin.test_email.body"), mailConfig, license != nil && *license.Features.Compliance, ""); err != nil {