server: clean up the attributes, relax the substring filter conditions

This consolidates both user and group attributes in their map_{user,group}_attribute as the only point of parsing. It adds support for custom attribute filters for groups, and makes a SubString filter on an unknown attribute resolve to just false.

server/src/domain/handler.rs

@@ -83,6 +83,7 @@ pub enum GroupRequestFilter {
     GroupId(GroupId),
     // Check if the group contains a user identified by uid.
     Member(UserId),
+    AttributeEquality(AttributeName, Serialized),
 }
 
 impl From<bool> for GroupRequestFilter {