Change submit url to target the specific member panel, to keep the URL consistent

[mircobabini]

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Changes proposed in this Pull Request:

Before this fix, when you change a Level manually, after saving you going to see that:

• You are still on the Member > Membership tab (because of the hidden field pmpro_member_edit_panel)
• The URL does NOT contain the pmpro_member_edit_panel anymore
• Refreshing the page will bring you away from the Member > Membership panel obviously

Moving the pmpro_member_edit_panel directive from the hidden field to the submit action will make the URL consistent after saving.

Other information:

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• 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.

[mircobabini]

We have a similar behavior after saving a level in Settings > Levels. Slightly different, but similar.