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 service.py file with API for registering/unregistering tools

- Created test_registration.py to test service.py

- 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

scmtool_name='Subversion'

- Refactor RepositoryForm to use scmtool_name

- Other misc refactoring to pass tests in scmtools, hostingsvcs,

webapi, reviews, and diffviewer

- Add migratescmtools.py command.

- Add SCMToolHook extension and unit tests

Currently passing all tests inscmtools, hostingsvcs, webapi,

reviews, and diffviewer.
Tested migratescmtools.py:

- 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.