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/slashcommands/command_msg_test.go

@@ -6,10 +6,10 @@ package slashcommands
 import (
 	"testing"
 
-	"github.com/mattermost/go-i18n/i18n"
 	"github.com/stretchr/testify/assert"
 
 	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v5/shared/i18n"
 )
 
 func TestMsgProvider(t *testing.T) {