add global unknown user

The point of this is to add a user to the DB that any orphaned data can be assigned to. this orphaned data may be as a result of a user being deleted or some other action. When this is done, the `user.account.delete` event is fired and modules can take appropriate action to reassign their data to the `unknown user`.

It is better to have this already done in the core because otherwise each module will create their own solution and there will be multiple 'unknown users' in the db. 

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | na
| Fixed tickets | 
| Refs tickets  | #1322
| License       | MIT
| Doc PR        |