possible fix for #1909
optimize getPostSummaryByPids
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.
using websocketAddress config if supplied
online users fix

and jshint cleanup
closes #1920
closes #1922
fix guest priv
closes #1924
logic for #1837
fix var name, #1925
Merge branch 'master' of https://github.com/NodeBB/NodeBB
small cleanup to postSummary
#1925
db search 0.0.13
fix fatal error when renaming group with no users
fluidized ACP groups modal dialog
Merge pull request #1928 from chronize/master

fix fatal error when renaming new group with no users
check err
group null and err check