Refactor SCM tool registration using a registry

Review Request #9724 — Created March 3, 2018 and discarded — Latest diff uploaded

Review Board
reviewboard, students

Instead of registering SCM tools in the database, populate a registry
at startup. This allows for more flexibility for users to register and
unregister external tools.

Things done:
- Rename keys in registry to match the names in SCM tools classes
- Created file with API for registering/unregistering tools
- Created to test
- Renamed Tool model to LegacyTool
- Created new class Tool which is not a model. Instantiated by passing
in an SCM tool name (string). This class has the same properties as
LegacyTool but gets the SCM tool class from the registry.
- Renamed tool field in Repository model to _legacy_tool and add a
scmtool_name field. Created an evolution for this.
- Created tool property in Repository which does a lazy migration
away from LegacyTool by checking the value of scmtool_name.
- Replace usages of the legacy tool to use scmtool_name
E.g. tool = LegacyTool.objects.get(name='Subversion') to
- Refactor RepositoryForm to use scmtool_name
- Other misc refactoring to pass tests in scmtools, hostingsvcs,
webapi, reviews, and diffviewer
- Add command.
- Add SCMToolHook extension and unit tests

Currently passing all tests inscmtools, hostingsvcs, webapi,
reviews, and diffviewer.

- Add local repos at /admin/db/scmtools/repository/add/
- Since the RepositoryForm uses scmtool_name now, manually
update repos in Django shell to be "pre-migration"
- Check that repo scmtool_name and _legacy_tool fields are as
expected post-migration

Unit tests for SCMToolHook extension.