Data Explorer queries have a `user_id` assigned to each query created. DE Reports can be bookmarked for later reference.
When creating the bookmark notification there was the possibility of a notification error being thrown (that made the notification menu inaccessible) due to a DE Query not having a owner (associated user_id). This can happen in a couple ways:
- having a query created by a user that was then later deleted leaving the query without ownership
- having a TA create a query for a customer using a temporary account, that would then later be deleted leaving the query without ownership
Since there is a case that `bookmark.user` is not valid the PR makes the `bookmark.user.username` optional for a bookmark notification. As [tested](https://github.com/discourse/discourse/pull/19851/files#diff-5b5154de37f96988d551feff6f1dfe5ba804fbcbc1c33b5478dde02a447a634f) in the case a username is not present, we will still render the `content` of the notification minus the username. This creates a safe fallback when looking up non-valid users.
* Use QUnit `module` instead of `discourseModule`
* Use QUnit `test` instead of `componentTest`
* Use angle-bracket syntax
* Remove jQuery usage
* Improve assertions (and actually fix some of them)
