Wrong keyboard navigation order for toggles and "x" icons on options page #3107

To reproduce:

1. Open PB's options page
2. Select the Tracking Domains tab
3. Press Tab to start keyboard navigation
4. Keep pressing Tab until the focus indicator goes into the list of tracking domains
5. Within each row in the list of tracking domains, Tab should focus the red/yellow/green toggle before the "x" (remove domain) icon, but the order is reversed instead.

Fixing this probably means replacing floats with a modern (flex?) layout.