optimize privileges and assorted fixes.

* new methods
privileges.categories.filter
privileges.topics.filter
privileges.posts.filter

they take a list of ids and a privilege, and return the filtered list of
ids, faster than doing  async.filter and calling the db for each id.

* remove event listeners on recent page before adding
* group.exists works for both single group names and arrays
* helpers.allowedTo works for both a single cid and an array of cids
* moved filter:topic.post hook right before topic creation.
* moved filter:topic.reply hook right before topic reply.