Extracting email functions into a service (#14802)

* Extracting email functions into a service

* Fixing two shadowing errors

* Address PR review comments

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/session.go

@@ -376,7 +376,7 @@ func (a *App) CreateUserAccessToken(token *model.UserAccessToken) (*model.UserAc
 
 	// Don't send emails to bot users.
 	if !user.IsBot {
-		if err := a.sendUserAccessTokenAddedEmail(user.Email, user.Locale, a.GetSiteURL()); err != nil {
+		if err := a.Srv().EmailService.sendUserAccessTokenAddedEmail(user.Email, user.Locale, a.GetSiteURL()); err != nil {
 			a.Log().Error("Unable to send user access token added email", mlog.Err(err), mlog.String("user_id", user.Id))
 		}
 	}