Add a management command to manage workspaces in a scope

Commands need to be easy to run from ansible, and so also idempotent.

Likely needed:

• create workspace
  • [./manage.py create_workspace already existed]
  • scope support for ./manage.py *_workspace (!1197 (merged))
  • add workspaces roles (#489 (closed) !1205 (merged))
  • create an Owners group for them, similar to what happens with Scope [scope create --with-owners-group, cf. #513 (closed) !1148 (merged)], but mandatory
  • make idempotent
• rename workspace
• set roles for groups on workspace
• handle naming conflicts between scope (!1218 (merged)) (!1197 (merged))
• update documentation (ongoing) (!1225 (merged))

(report time in #395 (closed))