[IMP] website: use HttpCaseWithUserDemo for tests and clean boostrapping

RATIONALE

Event will soon gain a major update called Event Online, allowing to better
support full-online events. In order to prepare its merge, preparatory merge
are done to lessen the final diff and have a smooth integration in a stable
version (13.3).

PURPOSE

Purpose of this merge is to clean visitor synchronization and tests. It
prepares further improvements to visitor model linked to Event Online

SPECIFICATIONS

Clean existing visitor tests. Use HttpCaseWithUserDemo and clean bootstrapping
of data.

Introduce a mock for visitor from request allowing to shortcut some visitor /
user synchronization and test directly expected results without too much
boilerplate in tests.

LINKS

Task ID 2290016 (improve visitor synchronization and tests)
Prepares Task ID 2252655 (main Online Event task)
Prepares Task ID 2284043 (Visitor-based track wishlist)
PR #54036

X-original-commit: 4e0d4e7c315b5a9d880754e02966debd6a59c728