[FIX] website_mail_channel: re-add import dropped by API migration

The migration to new API dropped `from openerp import tools`,
but it was still used.

See also 8250762fe8

addons/website_mail_channel/controllers/main.py

@@ -4,11 +4,10 @@
 from datetime import datetime
 from dateutil import relativedelta
 
-from odoo import http, fields
+from odoo import http, fields, tools
 from odoo.http import request
 from odoo.addons.website.models.website import slug
 
-
 class MailGroup(http.Controller):
     _thread_per_page = 20
     _replies_per_page = 10