Implement permission checks and stand-alone/pre-filled config checks

[swrichards]

As discussed on 31/3, the last step:

• Support specifying permissions
  • staff is a baseline, but we support other flavours too: IsSuperuser(), perhaps something related to the admin model/content type permissions, generic permission ('can_run_cases_config_check').
• Support for both standalone and pre-filled runs
  • The config check can be triggered as now on a specific object in the admin. But this flow should pre-fill data from the object where necessary (maybe through form initial fields)
  • We should be able to navigate directly to a config check, without being attached to a specific instance/object (And thus no pre-fill).