[FIX] mass_mailing: absolute URLs to email images

Partial backport of 9.0 commit a14f89c8 to 8.0
<base href=""> are not well supported in some webmails
Replace absolute URLs by full path appending the domain.

Closes #10062

addons/mass_mailing/models/mail_mail.py

@@ -21,6 +21,7 @@
 
 import urlparse
 import werkzeug.urls
+import re
 
 from openerp.tools.translate import _
 from openerp import tools
@@ -80,6 +81,14 @@ class MailMail(osv.Model):
         base = "<base href='%s'>" % domain
         body = tools.append_content_to_html(base, body, plaintext=False, container_tag='div')
 
+        # resolve relative image url to absolute for outlook.com
+        def _sub_relative2absolute(match):
+            return match.group(1) + urlparse.urljoin(domain, match.group(2))
+        # Regex: https://regex101.com/r/aE8uG5/3
+        body = re.sub('(<img(?=\s)[^>]*\ssrc=["\'])(/[^/][^"\']+)', _sub_relative2absolute, body)
+        # Regex: https://regex101.com/r/kT3lD5/2
+        body = re.sub(r'(<[^>]+\bstyle=["\'][^"\']+\burl\([\'"]?)(/[^/\'"][^\'")]+)', _sub_relative2absolute, body)
+
         # generate tracking URL
         if mail.statistics_ids:
             tracking_url = self._get_tracking_url(cr, uid, mail, partner, context=context)