[FIX] website_event - Check that template exists before to render it
[IMP] [BACKPORT] Mailing lists usability improvements: headers related to mailing lists, use of email queue for notification emails (> 50 followers), archive website template imp and fix. Backport of commit 839815f of saas-5 (PR 502).
Merge pull request #830 from odoo-dev/saas-4-mailing-lists-imp-tde

[IMP] [BACKPORT] Mailing lists usability improvements: headers related t...
[FIX] auth_signup: discard signup values that may overwriting existing info
[FIX] Website pager - Be sure that page in get param is a digit else use page 1.

Need to find which controller add ?page= instead of &page= because google find some url with '?page=1?page=2' --> page = '1?page=2'. According to google, the referrer is the old website.