[FIX] website_event: event dates within event timezone

The dates of an event on the website was always
within the user timezone. If not signed in,
it was within the public user timezone
(which is quite not user-friendly to change)

For this purpose, we weed get the context
from the record (the context used when using
`record.with_context`), and therefore
the QWeb field
`ir.qweb.field.datetime` is slightly altered,
to use the context from the record.

opw-675427