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_help.go

@@ -4,10 +4,9 @@
 package slashcommands
 
 import (
-	goi18n "github.com/mattermost/go-i18n/i18n"
-
 	"github.com/mattermost/mattermost-server/v5/app"
 	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v5/shared/i18n"
 )
 
 type HelpProvider struct {
@@ -25,7 +24,7 @@ func (h *HelpProvider) GetTrigger() string {
 	return CmdHelp
 }
 
-func (h *HelpProvider) GetCommand(a *app.App, T goi18n.TranslateFunc) *model.Command {
+func (h *HelpProvider) GetCommand(a *app.App, T i18n.TranslateFunc) *model.Command {
 	return &model.Command{
 		Trigger:          CmdHelp,
 		AutoComplete:     true,