Add Audit trail per challenge

[kikmann]

• [x] implement module for auditing
• [x] use in strava-cloudunc
• [x] use for user actions
• [ ] use for challenge admin actions
• [ ] use for admin actions
• [x] add audit trail page

[kikmann]

audit trail page only for me, not for admins

[kikmann]

maybe more to come, for the time being this is OK