fix(ads): move shared sticky ads styles to newspack-ads plugin

[laurelfulford]

All Submissions:

• Have you followed the Newspack Contributing guidelines?
• Does your code follow the WordPress coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

This PR moves some sticky footer ad styles from the classic theme to the Newspack Ads plugin, so they can be shared with the block theme.

I also moved any non-theme-specific or non-theme-dependant AMP ad styles.

Closes NPPD-1643

How to test the changes in this Pull Request:

1. Do a visual comparison of the classic theme on main and this branch, and confirm things look the same.
2. Confirm that no CSS was lost in this change (I tidied up the Newspack Ads CSS file a bit to reduce selector duplication).

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?
• Have you successfully run tests with your changes locally?