Auto-generate NTP backgrounds LICENSE file

[fmarier]

Fixes #7460 along with brave/brave-core#4481.

Submitter Checklist:

• 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).
• Verified that these changes build without errors on
  • Android
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that these changes pass automated tests (unit, browser, security tests) on
  • iOS
  • Linux
  • macOS
  • Windows
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Requested a security/privacy review as needed.
• If adding a new directory with NPM packages, ensure this directory is
  added to scripts/audit.js.
• Public documentation has been updated as necessary. For instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protection-Mode
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-compatibility-issues-with-tracking-protection

Test Plan:

npm run build Release

Look at the "Background images" entry in brave://credits. It should contain URLs and licenses for all 16 background images.

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions.

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on.
• All relevant documentation has been updated.

[bsclifton]

Left notes - LMK what you think! 😄

[fmarier]

The only test failure is this one:

14:31:41  2 tests failed:
14:31:41      AutoplayPermissionContextBrowserTest.AskAutoplay (../../brave/browser/autoplay/autoplay_permission_context_browsertest.cc:227)
14:31:41      AutoplayPermissionContextBrowserTest.BlockByDefault (../../brave/browser/autoplay/autoplay_permission_context_browsertest.cc:189)

which is a known problem.

@bsclifton @bridiver Can one of you re-review so that I can merge this PR?

[bsclifton]

++

[cezaraugusto]

cc @fmarier better if this could be case insensitive. had to workaround in brave/brave-core#4864