Disable ZODB hook in room occurrence task

That way we can safely commit after sending our emails in a try..finally
to avoid spamming people in case a notification fails (which causes the
scheduler worker to not commit anything)
Fix jinja unicode hack for inline if
Fix string_test
Add unittest for EnsureUnicodeExtension
Fix invalid reference
Reword book-prebook overlap warning message
Fix repetitions skipping 5th weekdays of months
Remove the threaded scheduler mode