Make the new membership level default expire date i18zed

[mircobabini]

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Changes proposed in this Pull Request:

Fixes the default expire date when manually assigning a new membership level, which is not adding the local offset.

1. Set your WP site to Europe/Rome for instance (that is +1 or +2).
2. Take a random member without a membership and add a new membership, setting to expire.
3. Check the local datetime input default value. The time will be GMT instead of 18zed (+1 year).

Please note: can't use wp_date bc we still support WP 5.2 compat and wp_date is 5.3+.

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Changelog entry

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