[FIX] Render of report shouldn't update context

With API 8 context is frozendict that don't support update
[FIX] website_mail: connected users can't follow a website document
[FIX] website_event_sale: display the discounted price in website.

Add price_reduce compute method on ticket and sale order line
[IMP] res_country: add relation from country to group_country. The purpose of this patch is to allow in domain (model.country_id.country_group_ids, '=', 'Europe')
[FIX] web: avoid incoherent actual_mode after refresh

When accessing an existing record in form mode directly (enter the url or refresh a page), the daterecord has not been initialized yet. This means that the value of actual_mode will be set to 'edit' before loading the current record (method _actualize_mode() called from do_show()).
This was problematic for one2many fields that we loaded in edit mode, showing add/delete icons/buttons in readonly views. (opw 607910)
[FIX] When using "--stop-after-init", set the return code correctly. A non zero return code reflect the number of databases that fail to load/update

Backport of 2c4e370b766db6280d38a4d3535e07de604b1922 and
33ce0e73dba7c23d004a278f61943be8052e7f37
[FIX] Report: use ir.config 'report.url' key as base url if it exists

Browse ir.config 'report.url' with superuser as
the user may want to restrict access of this key to the public
[IMP] config: Test that the config file is readable

Fixes #3237

cherry-pick of 8e72049
[MERGE] forward port of branch 7.0 up to d2fd05e
[ADD] web: add a route '/web/dbredirect' to ensure the connection to the right database via the get param db.
[MERGE] forward port of branch saas-3 up to 309ca4f
[FIX] mail: append messages at the right place

e6790ba introduced a bug where messages were always appended at the end
of the thread, potentially displaying them in the wrong order.
[FIX] event_sale: event tickets carried over on duplication
[FIX] event_sale: event name carried over in invoice line
[MERGE] forward port of branch 8.0 up to 92183e5
Merge branch 'master' of https://github.com/odoo/odoo into master-linux64