Adds Brave Ads launch notification for existing Rewards users

[ryanml]

Fixes: brave/brave-browser#2914
UI PR: brave/brave-ui#346

Note: This is only intended to be shown to existing users of Rewards who have not enabled Ads yet

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
  • Windows
  • macOS
  • Linux
• Verified that these changes pass automated tests (npm test brave_unit_tests && npm test brave_browser_tests) on
  • Windows
  • macOS
  • Linux
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone

Test Plan:

1. Use a <0.63 version of Brave to create a profile.
2. Using the older version, simply enable rewards and do nothing else
3. Create a copy of the profile appropriately titled Brave-Browser-Development
4. Build version from this PR (passing --brave-ads-debug as a flag, this reduces the time to delete notification from 7 days to 5 mins) and start Brave
5. Navigate to the panel and confirm that the Ads notification prompting the user to turn on ads is available
6. Confirm that click "Turn on ads" takes you to brave://rewards and dismisses the notification. (Note this will just navigate to the page, it will not automatically enable Ads)
7. Restart browser
8. Confirm that the notification is not available

Run through the above steps again, except after step 2, disable Rewards before copying the profile. Ensure that when upgrading the notification is not shown.

Also, test this with a clean profile. Opt-in to Rewards, confirm that you don't get the notification (opting in enables ads)

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

cc: @mandar-brave @jsecretan

[ryanml]

@bridiver naming updates made