Fix capability checks, update capability for post_restrictions

[dparker1005]

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Changes proposed in this Pull Request:

Fixes issue where most REST API endpoints would be restricted to pmpro_edit_members (see change from $permission = current_user_can( $permission ); to $permission = current_user_can( $permission_to_check );)

Also updates the post_restrictions endpoint to check whether the current user can edit the specific post being updated.

Alternative to #3238

Resolves #3215

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Changelog entry

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

[MaximilianoRicoTabo]

LGTM