[Brave News]: Don't show subscribe button until opted in to Brave News

[fallaciousreasoning]

Resolves brave/brave-browser#27057

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:

See issue

[stephendonner]

Verified PASSED using

Brave	1.48.32 Chromium: 108.0.5359.94 (Official Build) nightly (x86_64)
Revision	713576b895246504ccc6b92c2fb8ce2d60194074-refs/branch-heads/5359_71@{#3}
OS	macOS Version 11.7.1 (Build 20G918)

Steps:

1. installed 1.48.32
2. launched Brave
3. opened brave://flags
4. ensured there's no flag for the Brave News button
5. loaded theatlantic.com
6. ensured there's no Manage Subscriptions button visible in the URL bar
7. opened a new-tab page
8. ensured the Brave News opt-in page is still there
9. clicked on Customize
10. clicked on Brave News
11. ensured the Brave News opt-in page is still there
12. clicked on Show Brave News
13. returned to theatlantic.com tab
14. confirmed the Manage Subscriptions RSS-feed icon is in the URL bar

Confirmed the Manage Subscriptions RSS-feed icon isn't visible until opted-in to Brave News

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