Changing tipsMigrationAlert -> boxAlert, adding Ads variant

[ryanml]

Related issue: brave/brave-browser#2562

The tips alert component was too specific, now it is more general, and we can add a flavor for auto contribute now if needed.

https://brave-ui-dmhift3yt.now.sh

Changes

Test plan

Link / storybook path to visual changes

Integration

• Does this contain changes to src/components or src/

  • Will you publish to npm immediately after this PR, or wait until sometime in the future?
  • Incompatible API change to something existing (major version increase)
  • Adding new backwards-compatible functionality? (minor version increase)
  • Fixing a bug backwards-compatibly? (patch version increase)

• Does this contain changes to src/features for brave-core?

  • Are there non backwards-compatible changes required for brave-core? Do not merge until brave-core PR is approvable. Link to brave-core PR:
  • Will you create brave-core PR to update to this commit after it is merged?
  • Wants uplift to brave-core feature branch?
    • When uplift-approved, merge to brave-core-0.VV.x feature branch
    • Create additional brave-core PRs for each feature branch to update commit