Add focus ring to about page form elements

[jkup]

This came up in our most recent #design meeting but currently it is impossible for keyboard-only users to know which form element they are on because we don't display a visual change on focus state.

Some of this will be fixed by #5916 which switches us from span's to button tags (which have focus state!) but this ticket is for adding back the blue focus ring to select boxes, textarea's and input fields.

[jkup]

Also for this ticket we should check what Chrome does for bookmarks in the bookmarks toolbar. Specifically:

• Are they tababble from the urlbar input?
• Do they have focus rings?

[bsclifton]

Fixed with #6037 which was merged into branch 0.13.1-branch.

Test Plan

1. Open Brave
2. Go to Preferences
3. Start tabbing through the document. Make sure all buttons and inputs/selects have blue focus rings.
4. Then Add some bookmarks
5. Toggle on show bookmark bar
6. Click in the urlbar
7. Start tabbing through, make sure each bookmark also gets a blue focus ring

[bsclifton]

reverted here 2aa39bb

Let's re-open the issue with a better approach if we'd like to solve something like this. Thanks 😄