preferences: clear cache in a more clever way

Introduce something similar to "cache cookie", but more suitable for
preferences that depend both on the site configuration and user
preferences.

We store for each user a value in _preference_cache and we modify this
value each time user change his user preference. This value is then
used part of the a cache key.
The same mechanism happen with a global value for site and group
preferences. See _getCacheId docstring for more details.

This way, when one user modify their preference this does not reset
cache for every user.

Because we have proper invalidation, increase the case duration by using
erp5_ui_short instead of erp5_ui_long

This is applied to both preference methods and to templates documents
inside preferences.

test_system_preference_value_prefererred_clear_cache_disabled is updated
to use another preference name, because it was same as in a previous
test, so that we don't have test isolation problems.