The new SCMTools registry work caused a problem trying to build docs.

The issue was that we were using AppConfig.ready to populate the

registry, but doing so will do some database accesses (in order to pull

any legacy Tool instances that don't exist in entry points into the

registry). According to the documentation for ready, we should avoid

doing any DB queries here, precisely because it can affect running

management commands.
This change moves the registry population back into a handler for the

initializing signal, along with a comment explaining why.
While it's apparently not necessary for the docs build, I've also added

the new scmtool_id field to the fixtures for the docs DB.

Ran unit tests.
Built docs.