Buttons alignment on about:certerror and NoScript dialog

[luixxiul]

Describe the issue you encountered:
Buttons on about:certerror lacks proper margin.

This is a regression caused by a commit I submitted to the PR #5916. The commit has been squashed by @jkup.

cf:
43e8923#diff-fca7d0eef4fe1430dbdc3733b1fc5fdaL12 and
43e8923#diff-fca7d0eef4fe1430dbdc3733b1fc5fdaR11

This causes a regression on NoScipt dialog too.

Expected behavior:
Buttons should be aligned properly (margin should be set, space-between with flexbox, etc)

• Brave Version: 0.13.0 Preview5

• Steps to reproduce:

  1. Open https://expired.badssl.com/
  2. Click "Advanced settings"

• Screenshot if needed:

• Any related issues:

[luixxiul]

I created a PR to fix it into this:

[luixxiul]

Test Plan 1

1. Open https://expired.badssl.com/
2. Make sure the buttons have margins

Test Plan 2

1. Open https://jsfiddle.net/
2. Disable JavaScript with the shield
3. Click the NoScript icon
4. Make sure the text has margin-bottom