Concurrent access to _manifesto index

[lizrice]

There is a race condition where different people or entities make updates to the _manifesto image, (and that's a perfectly legitimate and likely scenario e.g. Ann updates seccomp profile, while Bob changes the contact details, while the CI system adds a new scan report). We need to be able to either

• atomically retrieve, and then update, the _manifesto file contents
• spot that it has changed since we downloaded it, and reject the update