As part of the effort towards creating a registry for SCMTools, we're working towards deprecating the old `Tool` model, which was what we had initially created to support multiple SCM types. This model was initially populated via a database fixture, and over the years we've moved to entry points, and (soon) to a registry for storing all of the available tools. If `Tool` is going away over time, we need a new way for Repository to keep track of what SCMTool it's supposed to use. This change adds a new field called `scmtool_id`, which will be the ID that can be looked up in the registry. Populating this field is a little complex because the existing Tool object only stores the SCMTool class name, not the ID. We therefore can't initialize the field from an evolution, even using custom SQL. The solution we've settled on is to wrap the upgrade process with something that will store a mapping of the existing ID relations, run any database evolutions, and then apply the mapping to populate the field. This should allow us to move that information from the Tool model (and associated SCMTool class) into the Repository model during the upgrade, even in the case where the evolutions being run delete the Tool model entirely. For now the code that builds that stored info uses the Tool model, but there's a note about how we'll need to update that to use hand-written SQL queries when the model instance gets deleted in the future. Testing Done: Set up a bunch of different repositories and ran `manage.py upgrade`. Saw that the new field was populated correctly.