Bump navbar username CSS media query from 960px to 1024px

This PR addresses #11702

After testing quite a few different scenarios against `v2.3.1`, I've concluded the breakpoint for hiding the username in the nav should be changed from:
```css
$desktop /* 960px */
``` 
to:
```css
$tabletL /* 1024px */
```

In my testing, a username of around 14 to 15 characters would trigger the layout issue pictured in #11702. This difference (`64px`) seems to better address the smaller space available on tablets (and some older desktops).