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/web_hub_test.go

@@ -15,12 +15,12 @@ import (
 
 	"github.com/gobwas/ws"
 	"github.com/gobwas/ws/wsutil"
-	goi18n "github.com/mattermost/go-i18n/i18n"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/mock"
 	"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/store/storetest/mocks"
 )
 
@@ -68,7 +68,7 @@ func registerDummyWebConn(t *testing.T, a *App, addr net.Addr, userID string) *W
 	c, _, _, err := ws.Dial(context.Background(), "ws://"+addr.String()+"/ws")
 	require.NoError(t, err)
 
-	wc := a.NewWebConn(c, *session, goi18n.IdentityTfunc(), "en")
+	wc := a.NewWebConn(c, *session, i18n.IdentityTfunc(), "en")
 	a.HubRegister(wc)
 	go wc.Pump()
 	return wc