MPP-3623 4-mask upsell pop-up for Free Relay users

[lloan]

This PR fixes MPP-3626

L10N: mozilla-l10n/fx-private-relay-l10n#167

New feature description

Confirmed: Implementation of the corner pop-up when free relay user hits that limit – this will appear on the dashboard

• Corner popup that shows up when a user hits their 4th mask. Goes away once dismissed.

• This only shows up for free users

• There is a specific image and messaging provided for regions where phone plans are available and where only emails are available. US & Non US

• This is behind a feature flag: four_mask_limit_upsell

• See spec in figma: How to upsell 5 mask limit

Screenshot (if applicable)

How to test

• four_mask_limit_upsell flag should be turned on
• must use a free account
• must have 4 masks set

Checklist (Definition of Done)

• Product Owner accepted the User Story (demo of functionality completed) or waived the privilege.
• All acceptance criteria are met.
• Jira ticket has been updated (if needed) to match changes made during the development process.
• I've added or updated relevant docs in the docs/ directory
• Jira ticket has been updated (if needed) with suggestions for QA when this PR is deployed to stage.
• All UI revisions follow the coding standards, and use Protocol tokens where applicable (see /frontend/src/styles/tokens.scss).
• Commits in this PR are minimal and have descriptive commit messages.
• l10n changes have been submitted to the l10n repository, if any.

[rafeerahman]

Looks good, was there a PR in L10n for the strings?

[lloan]

@rafeerahman updated ticket with l10n PR.