The common query methods for review requests, review groups, and

repositories had one parameter for specifying a LocalSite and another

(show_all_local_sites) for specifying if the query should be across

all LocalSites.
As we begin to add more methods for calculating LocalSite-related

stats, we have more cases where callers need to be able to provide a

LocalSite, the global site, or specify "across all sites". Keeping

this two-parameter approach just makes all this more complicated and

messy, so this change introduces a new approach.
There's now a LocalSite.ALL value that represents "across all sites".

Functions can optionally accept this in order to explicitly request this

kind of query.
This is just a simple object, and will fail if being passed into

queryset filters, which means we won't accidentally break anything if

passed in. Any function that accepts LocalSite.ALL will need to be

documented as explicitly supporting it.
Right now, support is available in ReviewRequestManager,

ReviewGroupManager, and RepositoryManager query methods. Some of the

new stat-calculation functions will soon take this as well.
The old show_all_local_sites flag still works, but will emit a

deprecation warning. All internal code has been updated to use

LocalSite.ALL instead.

All unit tests pass, with no related warnings.