Fix alignment of showSuppress message and buttons on messageBox

[luixxiul]

Closes #8598

Auditors:

Test Plan:

1. Open about:preferences#sync
2. Click "Reset Sync"
3. Click "Reset Sync" on the modal dialog
4. Click "Cancel"
5. Click "Reset Sync" again
6. Make sure there exists margin between the supress message and the buttons on the footer

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

[cezaraugusto]

++