ismember and valid apis for acls

[behroozbc]

• [x] Create a method that uses the flattened lists to check if a given userId is a member of an ACLId
• [x] Use this method to create an API called /acl/is-member that takes ACLId as query param (the userId is reterived from the auth token) and returns true or false.
• [x] Create another API called /acl/is-valid. This API takes ACLId as query param and returns true iff the ACL exists. We will use this API in the frontend while managing aclMemberIds on the create ACL page
• [x] API /acl/recompute-memberships that (re)populates the redis database. This API can only be called by members of sys-admin