This replaces the core logic from the registerscmtools management

command with a call to a new Tool.objects.register_from_entrypoints()

method. This method does the work of looking for any entrypoint-provided

SCMTools not already in the database, and registers them.
The new implementation executes fewer queries, and slims down the query

for the initial lookup of tools. The management command is also less

verbose, providing a single-line summary after registration instead of a

one line per tool.

Tested the registerscmtools command with registering 1 SCMTool, many

SCMTools, and no SCMTools.