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/webhub_fuzz.go

@@ -17,9 +17,9 @@ import (
 	"time"
 
 	"github.com/gorilla/websocket"
-	goi18n "github.com/mattermost/go-i18n/i18n"
 
 	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v5/shared/i18n"
 	"github.com/mattermost/mattermost-server/v5/testlib"
 )
 
@@ -87,7 +87,7 @@ func registerDummyWebConn(a *App, addr net.Addr, userID string) *WebConn {
 		panic(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