Review Board 5 introduced a LocalSite.ALL value, which could be used

to query state without considering a LocalSite value (as opposed to a

None value for the global site). This was a bit hard to type

correctly, and we ran the risk of different blocks of code attempting

their own typing.
This change introduces an AnyOrAllLocalSites type, which supports a

LocalSite instance, None, or ALL.
To make this work, ALL needed to be converted from a singleton object

to a value supported in Literal. The safest approach was to use an

enum (which won't unintentionally cast to a string or integer in a

query). So we now have a private enum with only an ALL value.

All unit tests pass.