Brave News Source Suggestions

[fallaciousreasoning]

Resolves brave/brave-browser#25563

Limitations

This PR isn't a complete implementation of suggestions, but I think it's a good first pass, and provides sensible suggestions. I'm documenting a few of the limitations here:

• Suggestions are based only on:
  a) Whether a source has been visited
  b) How similar a source is to subscribed sources.
  c) How similar a source is to visited sources.
• Only the users current locale is used for suggestions. In future we should be able to incrementally improve this.

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

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, 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-Protections
  • 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-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

1. Enable the brave://flags/#brave-news-v2 flag
2. Subscribe to the The Drive publisher
3. Reload the new tab page
4. In the Brave News dialog there should be a suggestions row. It should include (if the similarity matrix hasn't been changed), PopularMechanics, Forbes, Car and Driver. Suggestions may (or may not) be slightly different

[petemill]

This is working pretty nicely! Just some minor nits.

[petemill]

Perhaps either this function and/or GetVisitWeightings could do with some unit tests

[petemill]

I ended up fixing some of these minor nits, because I had everything open - hope you don't mind!