`._self` causes QL to crash

Summary: Eric originally reported this as a more complex QL expression, but the essence of the bug is that, if you say ._self in a QL expression (with nothing before the dot), the parser crashes.

This manifests to the user as a hang on a blank Querki page, so it's pretty unfortunate.

The problem is that it's trying to parse it as an OID, because of the leading dot, so OID.apply() is throwing an exception. We need to handle that more gracefully.

As I would expect, this isn't specific to _self, but it does require the underscore. That's because anything without the underscore is potentially parseable as base-36; the underscore makes it unparseable.