Users overview sorting can be inconsistent

## Descrioption

The UI sorting seems to be case insensitive, while the sorting logic in the backend seems to be case sensitive. This can lead to inconsistent results.

Let's say:
- you see 10 results in the users overview per page
- you have 10 users with a full name that starts with A-Z (page 1)
- you have 10 users with a full name that starts with a-z (page 2)

When you change the page size to e.g. 20, the order will change and e.g. "olaf example" will be displayed directly after "Olaf test"

* Graylog Version: 6.0.0 rc2