SCMTools are a very old part of Review Board, and existed long before

any modern registry mechanisms were put in place. We've used an

in-database representation, the Tool model, to manage the

registration. In some cases, we used its primary key to uniquely

identify the particular SCMTool within the system. In others, we used

the SCMTool.name attribute instead (in the API, for instance).
We've long known that we've wanted to replace the existing system with a

modern registry. While this change does not do that, this takes a baby

step by adding a SCMTool.scmtool_id attribute, which defines a unique

ID for the tool. If not populated, it will default to the key in the

EntryPoint that registered the tool.
This will allow us to more easily make references to the tool in places

like mapping dictionaries, and to start writing more future-proof code

that centers more around the SCMTool than the Tool. An upcoming

change will begin moving RepositoryForm over to this, as a precursor

to some larger-scale work that's needed.

Unit tests pass.
Tested along with upcoming changes to RepositoryForm.