Add submodule recursivity to scc check-milestone command

- Move main logic to standalone check_milestone() method
Add logic to build tag from release using get_tag_prefix()
Refactor PR milestone check/set and add permissions check
Add permissions check to scc check-labels
Rename scc unrebased-prs command as scc check-prs

- Rename corresponding unit/integration tests
- Fix command order in scc/main.py
Fix scc check-milestone tests
Fix permissions check in check-labels
Fix help text
Modify scc check-milestone to accept only release numbers as arguments

- Built the prefixed tag for the starting and ending points of the search
- Fix the corresponding integration test
Merge pull request #137 from sbesson/admin_tools

Admin tools