The tickets are reworked, made easier to print and have
their structure reworked for qr code addition (in ENT branch).
Also they are easier to download as buttons are added to do so.

1. Design

Tickets have their background removed in order to ease
their printing. More room is given to details, the design
is simplified.

  New SIMPLIFIED TICKET

The confirmation email will now contain a link "See tickets"
leading to a new version of the full page ticket, called
"simplified". It does not generate / download PDF but rather
leads to a rendered qweb html page, with design working
properly on different screen sizes. The design is a tweaked
version of the full_page ticket template. It allows user
to see their ticket directly without needing a pdf viewer.

2. Download Tickets Buttons

"Download / See Tickets" Buttons are added in
- Email Templates (Ticket and Badge)
- The SO confirmation (if payment). All tickets, per event.
- The registration confirmation (if no payment). All tickets
in the same PDF. They are for the same event.

In order to access tickets, a route event/tickets_dl is added
in the controller. It takes an event_id, registration_ids and
a hash based on those plus the database secret using tools.hmac.
The groundtruth hash is obtained for given registration ids in the
method _get_tickets_dl_access_hash on event.event. The route is
used in all places listed above. The hash must be valid and attendees
must exist.

No condition on payment is set in tickets_dl as we want the download
tickets link to work for non-free tickets as well at the end of the
sale process, and as the sale order could not be linked to any invoice
at that stage (if automatic invoices is not set in the settings),
registrations being 'not paid' at that point. This could be cleaned
in further work as different states are to be clarified.

The route must be public as it must work even for public users.
(from email or as public user registering on front-end)

3. Email Templates

In order to have similar mail templates for badge and tickets,
Registration Badge content is duplicated from Registration
Confirmation and slightly tweaked, replacing existing body.

Also, the info of the payment / order moved from full page ticket
to the body of the confirmation email.

4. Sending Tickets On Payment

In order to ease the process to find tickets when they
are linked to the sale order, they are now sent on payment.

When an invoice is paid, we check on potential linked
sale order lines if there are linked event registrations. If so,
a message is posted on the SO record, sent to the associated
partner, with pdf attachments: 1 per event, with all tickets.
A comment message is used so that the tikets and comment
can be found on the portal chatter of the SO as well, for portal
users.

PR ENT - https://github.com/odoo/enterprise/pull/41164
PR UPG - https://github.com/odoo/upgrade/pull/5007

Task-3061851