Add a "_system" user for use by scripts

Raphaël pointed out in !790 (comment 485502) that it's awkward to need a user for tasks being run from the administrative interface. This commit adds the concept of a "system" user which doesn't need an email address and is excluded by debusine-admin list_users and debusine-admin manage_users, and adds a _system user with an unusable password that can be used for automated tasks.