[REF] website_event_*: remove manual datetime patch

Use freezegun as it is more easy to use and allow to remove a lot of extra
boilerplate. Indeed manually patching datetime is a PITA as a lot of imports
have to be patched.

A custom patching has to be done for create_date, as it is done using a value
stored on the cursor (cr._now), based on SQL now. As it has nothing to do
with standard python library it cannot be patched using freezegun.

Task-2703285 (Event performance improvements)
Task-2703289 (Event testing and coverage)

Part-of: odoo/odoo#81068