Optimize lookup of org membership on prototype and perms APIs

Fixes a major slowdown when working with permissions under organizations with a lot of members

Fixes https://www.pivotaltracker.com/story/show/144076113

data/model/organization.py

@@ -106,15 +106,28 @@ def remove_organization_member(org, user_obj):
       TeamMember.delete().where(TeamMember.id << members).execute()
 
 
-def get_organization_member_set(orgname):
+def get_organization_member_set(org, include_robots=False, users_filter=None):
+  """ Returns the set of all member usernames under the given organization, with optional
+      filtering by robots and/or by a specific set of User objects.
+  """
   Org = User.alias()
   org_users = (User
                .select(User.username)
                .join(TeamMember)
                .join(Team)
-               .join(Org, on=(Org.id == Team.organization))
-               .where(Org.username == orgname)
+               .where(Team.organization == org)
                .distinct())
+
+  if not include_robots:
+    org_users = org_users.where(User.robot == False)
+
+  if users_filter is not None:
+    ids_list = [u.id for u in users_filter if u is not None]
+    if not ids_list:
+      return set()
+
+    org_users = org_users.where(User.id << ids_list)
+
   return {user.username for user in org_users}