Impacted versions:  All stable versions

Steps to reproduce:
1. Create a new mass mailing e-mail and add an unsubscribe button in that e-mail template
2. Make sure your `web.base.url` parameter is set in the format `http://www.yourwebsite.com` (without a '/' at the end)
3. Send out the mass mailing (not as a test e-mail but real e-mail!) and check the unsubscribe button in the recieved e-mail. You'll see that you get a correct unsubscribe link:
![image](https://user-images.githubusercontent.com/6352350/40048337-47f60340-5832-11e8-9f0f-b1e1bec1a085.png)
4. Go to Settings > Technical > Parameters > System Parameters and add a key `web.base.url.freeze` to make sure that your website URL won't be rewritten. Now change your `web.base.url` parameter from `http://www.yourwebsite.com` to `http://www.yourwebsite.com/`
P.S: If you copy the URL of your website in Google Chrome it always automatically adds '/' at the end of every web URL, so the `web.base.url.freeze` is only do demonstrate the same behaviour and to make sure the issue is easy reproducable.
5. Duplicate your mass mailing campagn and send this one out (not as a test e-mail but real e-mail!). You'll see that the unsubscribe link is not correctly generated:
![image](https://user-images.githubusercontent.com/6352350/40048498-b47ee7de-5832-11e8-9591-d0af3f5f38d2.png)


Current behavior: The unsubscribe button its link is not correctly generated when your `web.base.url` contains a `/` as last character. `http://www.odoo.com` would work, `http://www.odoo.com/` would not.
It is because of this line of code:
https://github.com/odoo/odoo/blob/19878d011f8abdc737b7b702c4c79632670d4b78/addons/mass_mailing/models/mail_mail.py#L102
This line of code will give an issue if the URL ends with `/` as it will do `http://yourwebsite.com` + `/'unsubscribe_from_list'`, which creates `http://yourwebsite.com//unsubscribe_from_list` instead of `http://yourwebsite.com/unsubscripe_from_list`, which then creates an unique URL

Expected behavior: both `http://yourwebsite.com` and `http://yourwebsite.com/` work for unsubscribing

addons/mass_mailing/models/mail_mail.py
import re
import urlparse
import werkzeug.urls
from openerp import tools
from openerp import SUPERUSER_ID
from openerp.osv import osv, fields
from openerp.addons.link_tracker.models.link_tracker import URL_REGEX
class MailMail(osv.Model):