Using default level and discount code filters

[dparker1005]

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• [x] Have you followed the Contributing guideline?
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Changes proposed in this Pull Request:

Should only be merged once this PR is confirmed to be merged into the PMPro v3.0 branch: https://github.com/strangerstudios/paid-memberships-pro/pull/2506

The PMPro pull request prefixes the level and discount_code URL parameters at checkout. Since Sitewide Sales programmatically sets the level and discount code, we need to make changes in this plugin to support PMPro v3.0.

This PR updates the automatic level/discount code logic to use filters instead of updating the $_REQEUST variables directly. In addition to adding compatibility with PMPro v3.0, this is also "the WordPress way" to do things.

Other information:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [x] Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.

[dparker1005]

Note: To test this, you must check out the PMPro 3.0 branch.