Add gift cards app

[gciotola]

Closes commercelayer/issues-app#173

What I did

• [x] app setup
• [x] listing page
• [x] details page
• [x] form page (edit / new)
• [x] activate draft gift card?
• [x] usage log/table in details page

Known issues:

• customer relationship is not created when creating a recipient (this means no cross link with app-customers until it's fixed)
• negative formatted currencies are shown as $-20.00 instead of -$20.00

How to test

Checklist

• [ ] Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests).
• [ ] Make sure to add/update documentation regarding your changes.
• [ ] You are NOT deprecating/removing a feature.

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