[major] Investigators & keys management

The management of investigators and  public GPG keys is finally improving! This pull request fully automates keyrings based on the content of the investigators table. It also adds tons of code to create, search and update investigators via the API and the console.

One slight security change is the scheduler private key that is now stored in the database. This has limited risks because only schedulers have the permission to access that column, and the key is only used to terminate old agents after an upgrade, such that a key leak would have no impact on the security of mig.