Update with the admin feature

[hug-dev]

After parallaxsecond/parsec#308 is implemented, the following things need to be updated:

• In the operations page we would add a paragraph about admins operations and in each of them (ListClients and DeleteClient), add the information that it is an admin operation.
• Add the new status code here (see Add a new ResponseStatus for admin operations parsec-interface-rs#93)

Threat model updates

• A9 for Spoofing and Tampering need to be updated with the new idea that someone malicious could set itself as the admin. More assets are then impacted because all clients keys can be destroyed. O-6 cover the right mitigation.
• A1 Spoofing threats are aggravated if the authentication stolen is the admin one: in that case not only admin keys are at risks but everyone's (only allows deletion of keys, not exposure/usage). I propose that we add a notice in the AS1 section noting down that the admin's authentication token is particularly sensitive as it impacts all.