Re-add newtab background images (as an option)

[bbondy]

Test plan

1. Launch Brave and visit about:preferences#tabs
2. Default setting should be true
3. Open new tab; verify backgrounds are shown
4. Close new tab and change setting to false
5. Open new tab; verify gradients are shown

Automated tests

npm run unittest -- --grep="NewTab component unit tests"

Original issue text

Because of high memory usage bug in chromium, background images on newtab page were removed here:
94a620a

This task is to track re-adding it, but this time as an option, and we can enable it by default once the memory problem is fixed.

[dschwank]

+1 would be great to have this as an option!

[dschwank]

Yeah - the switch is implemented and works fine! Thanks!