Update extensionsTab to the modified BEM style
Update extensionsTab to the modified BEM style

Also:
- Remove flex from the top container as displaying the footer on the bottom is inconsistent compared with the other tabs

Closes #10207

Test Plan:
1. Open `about:preferences#extensions`
2. Make sure the table is properly displayed
3. Scroll the page to the bottom
4. Make sure the information div has `margin-bottom: 40px`

luixxiul/browser-laptop,luixxiul/browser-laptop,diasdavid/browser-laptop,diasdavid/browser-laptop,diasdavid/browser-laptop,diasdavid/browser-laptop,luixxiul/browser-laptop,luixxiul/browser-laptop