Ignore capacity criterion if capacity is unknown
Don't show None capacity if it's unknown
Fix contextmanager default value not being falsy

This resulted in unexpected behavior such as access checks being
disabled because the code assumed it was rendering an offline website
even though it wasn't
Avoid ElementAlreadyInIndex when moving events

Someone might click the link that performs the move twice and in that
case the events are already in the target category. Those are now simply
skipped (if the source category is not the expected one).