Closes #7345

- extensions.less is deleted
- itemList.less is unlinked
- globalStyles.spacing.aboutPageDetailsPageWidth is introduced

Auditors:

Test Plan:
1. Open about:preferences#advanced
2. Disable Torrent Viewer
3. Restart the browser
4. Open about:extensions
5. Make sure Torrent Viewer is disabled (grayed out)
6. Enable Torrent Viewer
7. Make sure Torrent Viewer is enabled