[ticket/15266] Expand functionality of content_visibility
Added new function "is_visible", and replaced several immediate
uses of the above, including a single event in the new function
to handle change in all places consistently, and much simpler.
[ticket/15266] Fix events in content_visibility
Additional errors found. The event parameter 'timestamp' does
not exist, it is really called 'time'. Affects four events:
core.set_post_visibility_before_sql
core.set_post_visibility_after
core.set_topic_visibility_before_sql
core.set_topic_visibility_after
