POC: Focus trap in non-AMP

[adekbadek]

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Changes proposed in this Pull Request:

Closes #24.

Because of lack of AMP callbacks, this focus trap is activated in a somewhat hacky way, and there's no way to deactivate it – which results in a JS error being thrown after the campaign is dismissed.
Ideally, we should use the focus trap as part of the general non-AMP JS refactor (replacing AMP scripts with bespoke JS) – so this is a POC.

How to test the changes in this Pull Request:

1. Create an overlay campaign, view page in non-AMP mode
2. Observe the focus is trapped inside the modal by tabbing through the focusable elements

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
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