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/notification_test.go

@@ -11,6 +11,7 @@ import (
 	"github.com/stretchr/testify/require"
 
 	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v5/shared/i18n"
 	"github.com/mattermost/mattermost-server/v5/utils"
 )
 
@@ -1821,7 +1822,7 @@ func TestPostNotificationGetSenderName(t *testing.T) {
 		},
 		"system message": {
 			post:     &model.Post{Type: model.POST_SYSTEM_MESSAGE_PREFIX + "custom"},
-			expected: utils.T("system.message.name"),
+			expected: i18n.T("system.message.name"),
 		},
 		"overridden username": {
 			post:           overriddenPost,