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.go

@@ -16,8 +16,8 @@ import (
 
 	"github.com/mattermost/mattermost-server/v5/mlog"
 	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v5/shared/i18n"
 	"github.com/mattermost/mattermost-server/v5/store"
-	"github.com/mattermost/mattermost-server/v5/utils"
 	"github.com/mattermost/mattermost-server/v5/utils/markdown"
 )
 
@@ -245,7 +245,7 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 
 	// Check for channel-wide mentions in channels that have too many members for those to work
 	if int64(len(profileMap)) > *a.Config().TeamSettings.MaxNotificationsPerChannel {
-		T := utils.GetUserTranslations(sender.Locale)
+		T := i18n.GetUserTranslations(sender.Locale)
 
 		if mentions.HereMentioned {
 			a.SendEphemeralPost(
@@ -486,7 +486,7 @@ func (a *App) userAllowsEmail(user *model.User, channelMemberNotificationProps m
 }
 
 func (a *App) sendNoUsersNotifiedByGroupInChannel(sender *model.User, post *model.Post, channel *model.Channel, group *model.Group) {
-	T := utils.GetUserTranslations(sender.Locale)
+	T := i18n.GetUserTranslations(sender.Locale)
 	ephemeralPost := &model.Post{
 		UserId:    sender.Id,
 		RootId:    post.RootId,
@@ -586,7 +586,7 @@ func makeOutOfChannelMentionPost(sender *model.User, post *model.Post, outOfChan
 	ogUsers := model.UserSlice(outOfGroupsUsers)
 	ogUsernames := ogUsers.Usernames()
 
-	T := utils.GetUserTranslations(sender.Locale)
+	T := i18n.GetUserTranslations(sender.Locale)
 
 	ephemeralPostId := model.NewId()
 	var message string
@@ -1003,7 +1003,7 @@ func (n *PostNotification) GetChannelName(userNameFormat, excludeId string) stri
 // and whether or not the username has been overridden by an integration.
 func (n *PostNotification) GetSenderName(userNameFormat string, overridesAllowed bool) string {
 	if n.Post.IsSystemMessage() {
-		return utils.T("system.message.name")
+		return i18n.T("system.message.name")
 	}
 
 	if overridesAllowed && n.Channel.Type != model.CHANNEL_DIRECT {